//-----------------------------------------------------------------------------
//
// Copyright (C) Microsoft Corporation.  All Rights Reserved.
//
//-----------------------------------------------------------------------------
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading;
using System.IO;
using Microsoft.Boogie;
using Graphing;
using AI = Microsoft.AbstractInterpretationFramework;
using System.Diagnostics.Contracts;
using Microsoft.Basetypes;
using Microsoft.Boogie.VCExprAST;

namespace VC {
  using Bpl = Microsoft.Boogie;

  public class VCGen : ConditionGeneration {

    /// <summary>
    /// Constructor.  Initializes the theorem prover.
    /// </summary>
    [NotDelayed]
    public VCGen(Program program, string/*?*/ logFilePath, bool appendLogFile)
      : base(program)
      // throws ProverException
    {
      Contract.Requires(program != null);
      this.appendLogFile = appendLogFile;
      this.logFilePath = logFilePath;
      implName2LazyInliningInfo = new Dictionary<string, LazyInliningInfo>();

      if (CommandLineOptions.Clo.LazyInlining > 0) {
        this.GenerateVCsForLazyInlining(program);
      }
    }

    private static AssumeCmd AssertTurnedIntoAssume(AssertCmd assrt) {
      Contract.Requires(assrt != null);
      Contract.Ensures(Contract.Result<AssumeCmd>() != null);

      Expr expr = assrt.Expr;
      Contract.Assert(expr != null);
      switch (Wlp.Subsumption(assrt)) {
        case CommandLineOptions.SubsumptionOption.Never:
          expr = Expr.True;
          break;
        case CommandLineOptions.SubsumptionOption.Always:
          break;
        case CommandLineOptions.SubsumptionOption.NotForQuantifiers:
          if (expr is QuantifierExpr) {
            expr = Expr.True;
          }
          break;
        default:
          Contract.Assert(false);
          throw new cce.UnreachableException();  // unexpected case
      }

      return new AssumeCmd(assrt.tok, expr);
    }

    #region LazyInlining
    public class LazyInliningInfo {
      [ContractInvariantMethod]
      void ObjectInvariant() {
        Contract.Invariant(impl != null);
        Contract.Invariant(function != null);
        Contract.Invariant(controlFlowVariable != null);
        Contract.Invariant(assertExpr != null);
        Contract.Invariant(cce.NonNullElements(interfaceVars));
        Contract.Invariant(cce.NonNullElements(incarnationOriginMap));
      }

      public Implementation impl;
      public int uniqueId;
      public Function function;
      public Variable controlFlowVariable;
      public List<Variable> interfaceVars;
      public Expr assertExpr;
      public VCExpr vcexpr;
      public Dictionary<Incarnation, Absy> incarnationOriginMap;
      public Hashtable /*Variable->Expr*/ exitIncarnationMap;
      public Hashtable /*GotoCmd->returnCmd*/ gotoCmdOrigins;
      public Hashtable/*<int, Absy!>*/ label2absy;

      public LazyInliningInfo(Implementation impl, Program program, ProverContext ctxt, int uniqueId) {
        Contract.Requires(impl != null);
        Contract.Requires(program != null);
        this.impl = impl;
        this.uniqueId = uniqueId;
        this.controlFlowVariable = new LocalVariable(Token.NoToken, new TypedIdent(Token.NoToken, "cfc", Microsoft.Boogie.Type.Int));

        Procedure proc = cce.NonNull(impl.Proc);
        List<Variable> interfaceVars = new List<Variable>();
        Expr assertExpr = new LiteralExpr(Token.NoToken, true);
        Contract.Assert(assertExpr != null);
        foreach (Variable v in program.GlobalVariables()) {
          Contract.Assert(v != null);
          interfaceVars.Add(v);
        }
        // InParams must be obtained from impl and not proc
        foreach (Variable v in impl.InParams) {
          Contract.Assert(v != null);
          interfaceVars.Add(v);
        }
        // OutParams must be obtained from impl and not proc
        foreach (Variable v in impl.OutParams) {
          Contract.Assert(v != null);
          Constant c = new Constant(Token.NoToken,
                                    new TypedIdent(Token.NoToken, impl.Name + "_" + v.Name, v.TypedIdent.Type));
          interfaceVars.Add(c);
          Expr eqExpr = Expr.Eq(new IdentifierExpr(Token.NoToken, c), new IdentifierExpr(Token.NoToken, v));
          assertExpr = Expr.And(assertExpr, eqExpr);
        }
        foreach (IdentifierExpr e in proc.Modifies) {
          Contract.Assert(e != null);
          if (e.Decl == null)
            continue;
          Variable v = e.Decl;
          Constant c = new Constant(Token.NoToken,
                                    new TypedIdent(Token.NoToken, impl.Name + "_" + v.Name, v.TypedIdent.Type));
          interfaceVars.Add(c);
          Expr eqExpr = Expr.Eq(new IdentifierExpr(Token.NoToken, c), new IdentifierExpr(Token.NoToken, v));
          assertExpr = Expr.And(assertExpr, eqExpr);
        }
        this.interfaceVars = interfaceVars;
        this.assertExpr = Expr.Not(assertExpr);
        VariableSeq functionInterfaceVars = new VariableSeq();
        foreach (Variable v in interfaceVars) {
          Contract.Assert(v != null);
          functionInterfaceVars.Add(new Formal(Token.NoToken, new TypedIdent(Token.NoToken, v.Name, v.TypedIdent.Type), true));
        }
        TypedIdent ti = new TypedIdent(Token.NoToken, "", Bpl.Type.Bool);
        Contract.Assert(ti != null);
        Formal returnVar = new Formal(Token.NoToken, ti, false);
        Contract.Assert(returnVar != null);
        this.function = new Function(Token.NoToken, proc.Name, functionInterfaceVars, returnVar);
        ctxt.DeclareFunction(this.function, "");
      }
    }
    [ContractInvariantMethod]
    void ObjectInvariant() {
      Contract.Invariant(cce.NonNullElements(implName2LazyInliningInfo));
    }

    private Dictionary<string, LazyInliningInfo> implName2LazyInliningInfo;

    public void GenerateVCsForLazyInlining(Program program) {
      Contract.Requires(program != null);
      Checker checker = FindCheckerFor(null, CommandLineOptions.Clo.ProverKillTime);
      Contract.Assert(checker != null);

      VCExpr a = checker.VCExprGen.Integer(BigNum.ONE);
      VCExpr b = checker.VCExprGen.Integer(BigNum.ONE);
      VCExprNAry c = (VCExprNAry) checker.VCExprGen.ControlFlowFunctionApplication(a, b);
      VCExprBoogieFunctionOp op = (VCExprBoogieFunctionOp)c.Op;
      checker.TheoremProver.Context.DeclareFunction(op.Func, "");

      foreach (Declaration decl in program.TopLevelDeclarations) {
        Contract.Assert(decl != null);
        Implementation impl = decl as Implementation;
        if (impl == null)
          continue;
        Procedure proc = cce.NonNull(impl.Proc);
        if (proc.FindExprAttribute("inline") != null) {
          LazyInliningInfo info = new LazyInliningInfo(impl, program, checker.TheoremProver.Context, QuantifierExpr.GetNextSkolemId());
          implName2LazyInliningInfo[impl.Name] = info;
          impl.LocVars.Add(info.controlFlowVariable);
          ExprSeq exprs = new ExprSeq();
          foreach (Variable v in program.GlobalVariables()) {
            Contract.Assert(v != null);
            exprs.Add(new OldExpr(Token.NoToken, new IdentifierExpr(Token.NoToken, v)));
          }
          foreach (Variable v in proc.InParams) {
            Contract.Assert(v != null);
            exprs.Add(new IdentifierExpr(Token.NoToken, v));
          }
          foreach (Variable v in proc.OutParams) {
            Contract.Assert(v != null);
            exprs.Add(new IdentifierExpr(Token.NoToken, v));
          }
          foreach (IdentifierExpr ie in proc.Modifies) {
            Contract.Assert(ie != null);
            if (ie.Decl == null)
              continue;
            exprs.Add(ie);
          }
          Expr freePostExpr = new NAryExpr(Token.NoToken, new FunctionCall(info.function), exprs);
          proc.Ensures.Add(new Ensures(true, freePostExpr));
        }
      }

      foreach (LazyInliningInfo info in implName2LazyInliningInfo.Values) {
        Contract.Assert(info != null);
        GenerateVCForLazyInlining(program, info, checker);
      }
    }

    private void GenerateVCForLazyInlining(Program program, LazyInliningInfo info, Checker checker) {
      Contract.Requires(program != null);
      Contract.Requires(info != null);
      Contract.Requires(checker != null);
      Contract.Requires(info.impl != null);
      Contract.Requires(info.impl.Proc != null);

      Implementation impl = info.impl;
      ConvertCFG2DAG(impl, program);
      ModelViewInfo mvInfo;
      info.gotoCmdOrigins = PassifyImpl(impl, program, out mvInfo);
      Contract.Assert(info.exitIncarnationMap != null);
      Hashtable/*<int, Absy!>*/ label2absy;
      VCExpressionGenerator gen = checker.VCExprGen;
      Contract.Assert(gen != null);
      VCExpr vcexpr = gen.Not(GenerateVC(impl, info.controlFlowVariable, out label2absy, checker));
      Contract.Assert(vcexpr != null);
      info.label2absy = label2absy;

      Boogie2VCExprTranslator translator = checker.TheoremProver.Context.BoogieExprTranslator;
      Contract.Assert(translator != null);
      List<VCExprVar> privateVars = new List<VCExprVar>();
      foreach (Variable v in impl.LocVars) {
        Contract.Assert(v != null);
        privateVars.Add(translator.LookupVariable(v));
      }
      foreach (Variable v in impl.OutParams) {
        Contract.Assert(v != null);
        privateVars.Add(translator.LookupVariable(v));
      }
      if (privateVars.Count > 0) {
        vcexpr = gen.Exists(new List<TypeVariable>(), privateVars, new List<VCTrigger>(),
                            new VCQuantifierInfos(impl.Name, info.uniqueId, false, null), vcexpr);
      }

      List<VCExprVar> interfaceExprVars = new List<VCExprVar>();
      List<VCExpr> interfaceExprs = new List<VCExpr>();
      foreach (Variable v in info.interfaceVars) {
        Contract.Assert(v != null);
        VCExprVar ev = translator.LookupVariable(v);
        Contract.Assert(ev != null);
        interfaceExprVars.Add(ev);
        interfaceExprs.Add(ev);
      }

      Function function = cce.NonNull(info.function);
      VCExpr expr = gen.Function(function, interfaceExprs);
      Contract.Assert(expr != null);
      if (CommandLineOptions.Clo.LazyInlining == 1) {
        vcexpr = gen.Implies(expr, vcexpr);
      } else {
        Contract.Assert(CommandLineOptions.Clo.LazyInlining == 2);
        vcexpr = gen.Eq(expr, vcexpr);
      }

      List<VCTrigger> triggers = new List<VCTrigger>();
      List<VCExpr> exprs = new List<VCExpr>();
      exprs.Add(expr);
      VCTrigger trigger = new VCTrigger(true, exprs);
      Contract.Assert(trigger != null);
      triggers.Add(trigger);

      Expr e = new LiteralExpr(Token.NoToken, BigNum.FromInt(1));
      QKeyValue q = new QKeyValue(Token.NoToken, "weight", new List<object>(new object[] { e }), null);
      interfaceExprVars.Reverse();
      vcexpr = gen.Forall(new List<TypeVariable>(), interfaceExprVars, triggers,
                          new VCQuantifierInfos(impl.Name, QuantifierExpr.GetNextSkolemId(), false, q), vcexpr);

      info.vcexpr = vcexpr;
      checker.TheoremProver.PushVCExpression(vcexpr);
    }
    #endregion


    #region Soundness smoke tester
    class SmokeTester {
      [ContractInvariantMethod]
      void ObjectInvariant() {
        Contract.Invariant(parent != null);
        Contract.Invariant(impl != null);
        Contract.Invariant(initial != null);
        Contract.Invariant(program != null);
        Contract.Invariant(cce.NonNullElements(copies));
        Contract.Invariant(cce.NonNullElements(visited));
        Contract.Invariant(callback != null);
      }

      VCGen parent;
      Implementation impl;
      Block initial;
      Program program;
      int id;
      Dictionary<Block, Block> copies = new Dictionary<Block, Block>();
      Dictionary<Block, bool> visited = new Dictionary<Block, bool>();
      VerifierCallback callback;

      internal SmokeTester(VCGen par, Implementation i, Program prog, VerifierCallback callback) {
        Contract.Requires(par != null);
        Contract.Requires(i != null);
        Contract.Requires(prog != null);
        Contract.Requires(callback != null);
        parent = par;
        impl = i;
        initial = i.Blocks[0];
        program = prog;
        this.callback = callback;
      }

      internal void Copy() {
        CloneBlock(impl.Blocks[0]);
        initial = GetCopiedBlocks()[0];
      }

      internal void Test() {
        Contract.EnsuresOnThrow<UnexpectedProverOutputException>(true);

        DFS(initial);
      }

      void TopologicalSortImpl() {
        Graph<Block> dag = new Graph<Block>();
        dag.AddSource(cce.NonNull(impl.Blocks[0])); // there is always at least one node in the graph
        foreach (Block b in impl.Blocks) {
          GotoCmd gtc = b.TransferCmd as GotoCmd;
          if (gtc != null) {
            Contract.Assume(gtc.labelTargets != null);
            foreach (Block dest in gtc.labelTargets) {
              Contract.Assert(dest != null);
              dag.AddEdge(b, dest);
            }
          }
        }
        impl.Blocks = new List<Block>();
        foreach (Block b in dag.TopologicalSort()) {
          Contract.Assert(b != null);
          impl.Blocks.Add(b);
        }
      }

      void Emit() {
        TopologicalSortImpl();
        EmitImpl(impl, false);
      }

      // this one copies forward
      Block CloneBlock(Block b) {
        Contract.Requires(b != null);
        Contract.Ensures(Contract.Result<Block>() != null);

        Block fake_res;
        if (copies.TryGetValue(b, out fake_res)) {
          return cce.NonNull(fake_res);
        }
        Block res = new Block(b.tok, b.Label, new CmdSeq(b.Cmds), null);
        copies[b] = res;
        if (b.TransferCmd is GotoCmd) {
          foreach (Block ch in cce.NonNull((GotoCmd)b.TransferCmd).labelTargets) {
            Contract.Assert(ch != null);
            CloneBlock(ch);
          }
        }
        foreach (Block p in b.Predecessors) {
          Contract.Assert(p != null);
          res.Predecessors.Add(CloneBlock(p));
        }
        return res;
      }

      // this one copies backwards
      Block CopyBlock(Block b) {
        Contract.Requires(b != null);
        Contract.Ensures(Contract.Result<Block>() != null);

        Block fake_res;
        if (copies.TryGetValue(b, out fake_res)) {
          // fake_res should be Block! but the compiler fails
          return cce.NonNull(fake_res);
        }
        Block res;
        CmdSeq seq = new CmdSeq();
        foreach (Cmd c in b.Cmds) {
          Contract.Assert(c != null);
          AssertCmd turn = c as AssertCmd;
          if (!turnAssertIntoAssumes || turn == null) {
            seq.Add(c);
          } else {
            seq.Add(AssertTurnedIntoAssume(turn));
          }
        }
        res = new Block(b.tok, b.Label, seq, null);
        copies[b] = res;
        foreach (Block p in b.Predecessors) {
          Contract.Assert(p != null);
          res.Predecessors.Add(CopyBlock(p));
        }
        return res;
      }

      List<Block> GetCopiedBlocks() {
        Contract.Ensures(cce.NonNullElements(Contract.Result<List<Block>>()));

        // the order of nodes in res is random (except for the first one, being the entry)
        List<Block> res = new List<Block>();
        res.Add(copies[initial]);

        foreach (KeyValuePair<Block, Block> kv in copies) {
          Contract.Assert(kv.Key != null&&kv.Value!=null);
          GotoCmd go = kv.Key.TransferCmd as GotoCmd;
          ReturnCmd ret = kv.Key.TransferCmd as ReturnCmd;
          if (kv.Key != initial) {
            res.Add(kv.Value);
          }
          if (go != null) {
            GotoCmd copy = new GotoCmd(go.tok, new StringSeq(), new BlockSeq());
            kv.Value.TransferCmd = copy;
            foreach (Block b in cce.NonNull(go.labelTargets)) {
              Contract.Assert(b != null);
              Block c;
              if (copies.TryGetValue(b, out c)) {
                copy.AddTarget(cce.NonNull(c));
              }
            }
          } else if (ret != null) {
            kv.Value.TransferCmd = ret;
          } else {
            Contract.Assume(false);
            throw new cce.UnreachableException();
          }
        }

        copies.Clear();

        return res;
      }

      // check if e is true, false, !true, !false
      // if so return true and the value of the expression in val
      bool BooleanEval(Expr e, ref bool val) {
        Contract.Requires(e != null);
        LiteralExpr lit = e as LiteralExpr;
        NAryExpr call = e as NAryExpr;

        if (lit != null && lit.isBool) {
          val = lit.asBool;
          return true;
        } else if (call != null &&
                   call.Fun is UnaryOperator &&
                   ((UnaryOperator)call.Fun).Op == UnaryOperator.Opcode.Not &&
                   BooleanEval(cce.NonNull(call.Args[0]), ref val)) {
          val = !val;
          return true;
        }
          // this is for the 0bv32 != 0bv32 generated by vcc
        else if (call != null &&
                   call.Fun is BinaryOperator &&
                   ((BinaryOperator)call.Fun).Op == BinaryOperator.Opcode.Neq &&
                   call.Args[0] is LiteralExpr &&
                   cce.NonNull(call.Args[0]).Equals(call.Args[1])) {
          val = false;
          return true;
        }

        return false;
      }

      bool IsFalse(Expr e) {
        Contract.Requires(e != null);
        bool val = false;
        return BooleanEval(e, ref val) && !val;
      }

      bool CheckUnreachable(Block cur, CmdSeq seq) {
        Contract.Requires(cur != null);
        Contract.Requires(seq != null);
        Contract.EnsuresOnThrow<UnexpectedProverOutputException>(true);
        foreach (Cmd cmd in seq) {
          AssertCmd assrt = cmd as AssertCmd;
          if (assrt != null && QKeyValue.FindBoolAttribute(assrt.Attributes, "PossiblyUnreachable"))
            return false;
        }

        DateTime start = DateTime.Now;
        if (CommandLineOptions.Clo.Trace) {
          System.Console.Write("    soundness smoke test #{0} ... ", id);
        }
        callback.OnProgress("smoke", id, id, 0.0);

        Token tok = new Token();
        tok.val = "soundness smoke test assertion";
        seq.Add(new AssertCmd(tok, Expr.False));
        Block copy = CopyBlock(cur);
        Contract.Assert(copy != null);
        copy.Cmds = seq;
        List<Block> backup = impl.Blocks;
        Contract.Assert(backup != null);
        impl.Blocks = GetCopiedBlocks();
        copy.TransferCmd = new ReturnCmd(Token.NoToken);
        if (CommandLineOptions.Clo.TraceVerify) {
          System.Console.WriteLine();
          System.Console.WriteLine(" --- smoke #{0}, before passify", id);
          Emit();
        }
        parent.CurrentLocalVariables = impl.LocVars;
        ModelViewInfo mvInfo;
        parent.PassifyImpl(impl, program, out mvInfo);
        Hashtable label2Absy;
        Checker ch = parent.FindCheckerFor(impl, CommandLineOptions.Clo.SmokeTimeout);
        Contract.Assert(ch != null);
        VCExpr vc = parent.GenerateVC(impl, null, out label2Absy, ch);
        Contract.Assert(vc != null);
        impl.Blocks = backup;

        if (CommandLineOptions.Clo.TraceVerify) {
          System.Console.WriteLine(" --- smoke #{0}, after passify", id);
          Emit();
        }
        ch.BeginCheck(cce.NonNull(impl.Name + "_smoke" + id++), vc, new ErrorHandler(label2Absy, this.callback));
        ch.ProverDone.WaitOne();
        ProverInterface.Outcome outcome = ch.ReadOutcome();
        parent.CurrentLocalVariables = null;

        DateTime end = DateTime.Now;
        TimeSpan elapsed = end - start;
        if (CommandLineOptions.Clo.Trace) {
          System.Console.WriteLine("  [{0} s] {1}", elapsed.TotalSeconds,
            outcome == ProverInterface.Outcome.Valid ? "OOPS" :
              "OK" + (outcome == ProverInterface.Outcome.Invalid ? "" : " (" + outcome + ")"));
        }

        if (outcome == ProverInterface.Outcome.Valid) {
          // copy it again, so we get the version with calls, assignments and such
          copy = CopyBlock(cur);
          copy.Cmds = seq;
          impl.Blocks = GetCopiedBlocks();
          TopologicalSortImpl();
          callback.OnUnreachableCode(impl);
          impl.Blocks = backup;
          return true;
        }
        return false;
      }

      const bool turnAssertIntoAssumes = false;

      void DFS(Block cur) {
        Contract.Requires(cur != null);
        Contract.EnsuresOnThrow<UnexpectedProverOutputException>(true);
        if (visited.ContainsKey(cur))
          return;
        visited[cur] = true;

        CmdSeq seq = new CmdSeq();
        foreach (Cmd cmd_ in cur.Cmds) {
          Cmd cmd = cmd_;
          Contract.Assert(cmd != null);
          AssertCmd assrt = cmd as AssertCmd;
          AssumeCmd assm = cmd as AssumeCmd;
          CallCmd call = cmd as CallCmd;

          bool assumeFalse = false;

          if (assrt != null) {
            // we're not going any further
            // it's clear the user expected unreachable code here
            // it's not clear where did he expect it, maybe it would be right to insert
            // a check just one command before
            if (IsFalse(assrt.Expr))
              return;

#if TURN_ASSERT_INFO_ASSUMES
            if (turnAssertIntoAssumes) {
              cmd = AssertTurnedIntoAssume(assrt);
            }
#endif
          } else if (assm != null) {
            if (IsFalse(assm.Expr))
              assumeFalse = true;
          } else if (call != null) {
            foreach (Ensures e in (cce.NonNull(call.Proc)).Ensures) {
              Contract.Assert(e != null);
              if (IsFalse(e.Condition))
                assumeFalse = true;
            }
          }

          if (assumeFalse) {
            CheckUnreachable(cur, seq);
            return;
          }

          seq.Add(cmd);
        }


        GotoCmd go = cur.TransferCmd as GotoCmd;
        ReturnCmd ret = cur.TransferCmd as ReturnCmd;

        Contract.Assume(!(go != null && go.labelTargets == null && go.labelNames != null && go.labelNames.Length > 0));

        if (ret != null || (go != null && cce.NonNull(go.labelTargets).Length == 0)) {
          // we end in return, so there will be no more places to check
          CheckUnreachable(cur, seq);
        } else if (go != null) {
          bool needToCheck = true;
          // if all of our children have more than one parent, then
          // we're in the right place to check
          foreach (Block target in cce.NonNull(go.labelTargets)) {
            Contract.Assert(target != null);
            if (target.Predecessors.Length == 1) {
              needToCheck = false;
            }
          }
          if (needToCheck) {
            CheckUnreachable(cur, seq);
          }
          foreach (Block target in go.labelTargets) {
            Contract.Assert(target != null);
            DFS(target);
          }
        }
      }

      class ErrorHandler : ProverInterface.ErrorHandler {
        Hashtable label2Absy;
        VerifierCallback callback;
        [ContractInvariantMethod]
        void ObjectInvariant() {
          Contract.Invariant(label2Absy != null);
          Contract.Invariant(callback != null);
        }


        public ErrorHandler(Hashtable label2Absy, VerifierCallback callback) {
          Contract.Requires(label2Absy != null);
          Contract.Requires(callback != null);
          this.label2Absy = label2Absy;
          this.callback = callback;
        }

        public override Absy Label2Absy(string label) {
          //Contract.Requires(label != null);
          Contract.Ensures(Contract.Result<Absy>() != null);

          int id = int.Parse(label);
          return cce.NonNull((Absy)label2Absy[id]);
        }

        public override void OnProverWarning(string msg) {
          //Contract.Requires(msg != null);
          this.callback.OnWarning(msg);
        }
      }
    }


    #endregion

    #region Splitter
    class Split {
      class BlockStats {
        public bool big_block;
        public int id;
        public double assertion_cost;
        public double assumption_cost; // before multiplier
        public double incomming_paths;
        public List<Block>/*!>!*/ virtual_successors = new List<Block>();
        public List<Block>/*!>!*/ virtual_predecesors = new List<Block>();
        public Dictionary<Block, bool> reachable_blocks;
        public readonly Block block;
        [ContractInvariantMethod]
        void ObjectInvariant() {
          Contract.Invariant(cce.NonNullElements(virtual_successors));
          Contract.Invariant(cce.NonNullElements(virtual_predecesors));
          Contract.Invariant(cce.NonNullElements(reachable_blocks.Keys));
          Contract.Invariant(block != null);
        }


        public BlockStats(Block b, int i) {
          Contract.Requires(b != null);
          block = b;
          assertion_cost = -1;
          id = i;
        }
      }
      [ContractInvariantMethod]
      void ObjectInvariant() {
        Contract.Invariant(cce.NonNullElements(blocks));
        Contract.Invariant(cce.NonNullElements(big_blocks));
        Contract.Invariant(cce.NonNullElements(stats));
        Contract.Invariant(cce.NonNullElements(assumized_branches));
        Contract.Invariant(gotoCmdOrigins != null);
        Contract.Invariant(parent != null);
        Contract.Invariant(impl != null);
        Contract.Invariant(copies != null);
        Contract.Invariant(cce.NonNullElements(protected_from_assert_to_assume));
        Contract.Invariant(cce.NonNullElements(keep_at_all));
      }


      readonly List<Block> blocks;
      readonly List<Block> big_blocks = new List<Block>();
      readonly Dictionary<Block/*!*/, BlockStats/*!*/>/*!*/ stats = new Dictionary<Block/*!*/, BlockStats/*!*/>();
      readonly int id;
      static int current_id;
      Block split_block;
      bool assert_to_assume;
      List<Block/*!*/>/*!*/ assumized_branches = new List<Block/*!*/>();

      double score;
      bool score_computed;
      double total_cost;
      int assertion_count;
      double assertion_cost; // without multiplication by paths
      Hashtable/*TransferCmd->ReturnCmd*//*!*/ gotoCmdOrigins;
      VCGen/*!*/ parent;
      Implementation/*!*/ impl;

      Dictionary<Block/*!*/, Block/*!*/>/*!*/ copies = new Dictionary<Block/*!*/, Block/*!*/>();
      bool doing_slice;
      double slice_initial_limit;
      double slice_limit;
      bool slice_pos;
      Dictionary<Block/*!*/, bool>/*!*/ protected_from_assert_to_assume = new Dictionary<Block/*!*/, bool>();
      Dictionary<Block/*!*/, bool>/*!*/ keep_at_all = new Dictionary<Block/*!*/, bool>();

      // async interface
      private Checker checker;
      private int splitNo;
      internal ErrorReporter reporter;

      public Split(List<Block/*!*/>/*!*/ blocks, Hashtable/*TransferCmd->ReturnCmd*//*!*/ gotoCmdOrigins, VCGen/*!*/ par, Implementation/*!*/ impl) {
        Contract.Requires(cce.NonNullElements(blocks));
        Contract.Requires(gotoCmdOrigins != null);
        Contract.Requires(par != null);
        Contract.Requires(impl != null);
        this.blocks = blocks;
        this.gotoCmdOrigins = gotoCmdOrigins;
        this.parent = par;
        this.impl = impl;
        this.id = current_id++;
      }

      public double Cost {
        get {
          ComputeBestSplit();
          return total_cost;
        }
      }

      public bool LastChance {
        get {
          ComputeBestSplit();
          return assertion_count == 1 && score < 0;
        }
      }

      public string Stats {
        get {
          ComputeBestSplit();
          return string.Format("(cost:{0:0}/{1:0}{2})", total_cost, assertion_cost, LastChance ? " last" : "");
        }
      }

      public void DumpDot(int no) {
        using (System.IO.StreamWriter sw = System.IO.File.CreateText(string.Format("split.{0}.dot", no))) {
          sw.WriteLine("digraph G {");

          ComputeBestSplit();
          List<Block> saved = assumized_branches;
          Contract.Assert(saved != null);
          assumized_branches = new List<Block>();
          DoComputeScore(false);
          assumized_branches = saved;

          foreach (Block b in big_blocks) {
            Contract.Assert(b != null);
            BlockStats s = GetBlockStats(b);
            foreach (Block t in s.virtual_successors) {
              Contract.Assert(t != null);
              sw.WriteLine("n{0} -> n{1};", s.id, GetBlockStats(t).id);
            }
            sw.WriteLine("n{0} [label=\"{1}:\\n({2:0.0}+{3:0.0})*{4:0.0}\"{5}];",
                      s.id, b.Label,
                      s.assertion_cost, s.assumption_cost, s.incomming_paths,
                      s.assertion_cost > 0 ? ",shape=box" : "");

          }
          sw.WriteLine("}");
          sw.Close();
        }

        string filename = string.Format("split.{0}.bpl", no);
        using (System.IO.StreamWriter sw = System.IO.File.CreateText(filename)) {
          int oldPrintUnstructured = CommandLineOptions.Clo.PrintUnstructured;
          CommandLineOptions.Clo.PrintUnstructured = 2;  // print only the unstructured program
          bool oldPrintDesugaringSetting = CommandLineOptions.Clo.PrintDesugarings;
          CommandLineOptions.Clo.PrintDesugarings = false;
          List<Block> backup = impl.Blocks;
          Contract.Assert(backup != null);
          impl.Blocks = blocks;
          impl.Emit(new TokenTextWriter(filename, sw, false), 0);
          impl.Blocks = backup;
          CommandLineOptions.Clo.PrintDesugarings = oldPrintDesugaringSetting;
          CommandLineOptions.Clo.PrintUnstructured = oldPrintUnstructured;
        }
      }

      int bsid;
      BlockStats GetBlockStats(Block b) {
        Contract.Requires(b != null);
        Contract.Ensures(Contract.Result<BlockStats>() != null);

        BlockStats s;
        if (!stats.TryGetValue(b, out s)) {
          s = new BlockStats(b, bsid++);
          stats[b] = s;
        }
        return cce.NonNull(s);
      }

      double AssertionCost(PredicateCmd c) {
        return 1.0;
      }

      void CountAssertions(Block b) {
        Contract.Requires(b != null);
        BlockStats s = GetBlockStats(b);
        if (s.assertion_cost >= 0)
          return; // already done
        s.big_block = true;
        s.assertion_cost = 0;
        s.assumption_cost = 0;
        foreach (Cmd c in b.Cmds) {
          if (c is AssertCmd) {
            double cost = AssertionCost((AssertCmd)c);
            s.assertion_cost += cost;
            assertion_count++;
            assertion_cost += cost;
          } else if (c is AssumeCmd) {
            s.assumption_cost += AssertionCost((AssumeCmd)c);
          }
        }
        foreach (Block c in Exits(b)) {
          Contract.Assert(c != null);
          s.virtual_successors.Add(c);
        }
        if (s.virtual_successors.Count == 1) {
          Block next = s.virtual_successors[0];
          BlockStats se = GetBlockStats(next);
          CountAssertions(next);
          if (next.Predecessors.Length > 1 || se.virtual_successors.Count != 1)
            return;
          s.virtual_successors[0] = se.virtual_successors[0];
          s.assertion_cost += se.assertion_cost;
          s.assumption_cost += se.assumption_cost;
          se.big_block = false;
        }
      }

      Dictionary<Block/*!*/, bool>/*!*/ ComputeReachableNodes(Block/*!*/ b) {
        Contract.Requires(b != null);
        Contract.Ensures(cce.NonNullElements(Contract.Result<Dictionary<Block, bool>>()));
        BlockStats s = GetBlockStats(b);
        if (s.reachable_blocks != null) {
          return s.reachable_blocks;
        }
        Dictionary<Block/*!*/, bool> blocks = new Dictionary<Block/*!*/, bool>();
        s.reachable_blocks = blocks;
        blocks[b] = true;
        foreach (Block/*!*/ succ in Exits(b)) {
          Contract.Assert(succ != null);
          foreach (Block r in ComputeReachableNodes(succ).Keys) {
            Contract.Assert(r != null);
            blocks[r] = true;
          }
        }
        return blocks;
      }

      double ProverCost(double vc_cost) {
        return vc_cost * vc_cost;
      }

      void ComputeBestSplit() {
        if (score_computed)
          return;
        score_computed = true;

        assertion_count = 0;

        foreach (Block b in blocks) {
          Contract.Assert(b != null);
          CountAssertions(b);
        }

        foreach (Block b in blocks) {
          Contract.Assert(b != null);
          BlockStats bs = GetBlockStats(b);
          if (bs.big_block) {
            big_blocks.Add(b);
            foreach (Block ch in bs.virtual_successors) {
              Contract.Assert(ch != null);
              BlockStats chs = GetBlockStats(ch);
              if (!chs.big_block) {
                Console.WriteLine("non-big {0} accessed from {1}", ch, b);
                DumpDot(-1);
                Contract.Assert(false);
                throw new cce.UnreachableException();
              }
              chs.virtual_predecesors.Add(b);
            }
          }
        }

        assumized_branches.Clear();
        total_cost = ProverCost(DoComputeScore(false));

        score = double.PositiveInfinity;
        Block best_split = null;
        List<Block> saved_branches = new List<Block>();

        foreach (Block b in big_blocks) {
          Contract.Assert(b != null);
          GotoCmd gt = b.TransferCmd as GotoCmd;
          if (gt == null)
            continue;
          BlockSeq targ = cce.NonNull(gt.labelTargets);
          if (targ.Length < 2)
            continue;
          // caution, we only consider two first exits

          double left0, right0, left1, right1;
          split_block = b;

          assumized_branches.Clear();
          assumized_branches.Add(cce.NonNull(targ[0]));
          left0 = DoComputeScore(true);
          right0 = DoComputeScore(false);

          assumized_branches.Clear();
          for (int idx = 1; idx < targ.Length; idx++) {
            assumized_branches.Add(cce.NonNull(targ[idx]));
          }
          left1 = DoComputeScore(true);
          right1 = DoComputeScore(false);

          double current_score = ProverCost(left1) + ProverCost(right1);
          double other_score = ProverCost(left0) + ProverCost(right0);

          if (other_score < current_score) {
            current_score = other_score;
            assumized_branches.Clear();
            assumized_branches.Add(cce.NonNull(targ[0]));
          }

          if (current_score < score) {
            score = current_score;
            best_split = split_block;
            saved_branches.Clear();
            saved_branches.AddRange(assumized_branches);
          }
        }

        if (CommandLineOptions.Clo.VcsPathSplitMult * score > total_cost) {
          split_block = null;
          score = -1;
        } else {
          assumized_branches = saved_branches;
          split_block = best_split;
        }
      }

      void UpdateIncommingPaths(BlockStats s) {
        Contract.Requires(s != null);
        if (s.incomming_paths < 0.0) {
          int count = 0;
          s.incomming_paths = 0.0;
          if (!keep_at_all.ContainsKey(s.block))
            return;
          foreach (Block b in s.virtual_predecesors) {
            Contract.Assert(b != null);
            BlockStats ch = GetBlockStats(b);
            Contract.Assert(ch != null);
            UpdateIncommingPaths(ch);
            if (ch.incomming_paths > 0.0) {
              s.incomming_paths += ch.incomming_paths;
              count++;
            }
          }
          if (count > 1) {
            s.incomming_paths *= CommandLineOptions.Clo.VcsPathJoinMult;
          }
        }
      }

      void ComputeBlockSetsHelper(Block b, bool allow_small) {
        Contract.Requires(b != null);
        if (keep_at_all.ContainsKey(b))
          return;
        keep_at_all[b] = true;

        if (allow_small) {
          foreach (Block ch in Exits(b)) {
            Contract.Assert(ch != null);
            if (b == split_block && assumized_branches.Contains(ch))
              continue;
            ComputeBlockSetsHelper(ch, allow_small);
          }
        } else {
          foreach (Block ch in GetBlockStats(b).virtual_successors) {
            Contract.Assert(ch != null);
            if (b == split_block && assumized_branches.Contains(ch))
              continue;
            ComputeBlockSetsHelper(ch, allow_small);
          }
        }
      }

      void ComputeBlockSets(bool allow_small) {
        protected_from_assert_to_assume.Clear();
        keep_at_all.Clear();

        Debug.Assert(split_block == null || GetBlockStats(split_block).big_block);
        Debug.Assert(GetBlockStats(blocks[0]).big_block);

        if (assert_to_assume) {
          foreach (Block b in allow_small ? blocks : big_blocks) {
            Contract.Assert(b != null);
            if (ComputeReachableNodes(b).ContainsKey(cce.NonNull(split_block))) {
              keep_at_all[b] = true;
            }
          }

          foreach (Block b in assumized_branches) {
            Contract.Assert(b != null);
            foreach (Block r in ComputeReachableNodes(b).Keys) {
              Contract.Assert(r != null);
              if (allow_small || GetBlockStats(r).big_block) {
                keep_at_all[r] = true;
                protected_from_assert_to_assume[r] = true;
              }
            }
          }
        } else {
          ComputeBlockSetsHelper(blocks[0], allow_small);
        }
      }

      bool ShouldAssumize(Block b) {
        Contract.Requires(b != null);
        return assert_to_assume && !protected_from_assert_to_assume.ContainsKey(b);
      }

      double DoComputeScore(bool aa) {
        assert_to_assume = aa;
        ComputeBlockSets(false);

        foreach (Block b in big_blocks) {
          Contract.Assert(b != null);
          GetBlockStats(b).incomming_paths = -1.0;
        }

        GetBlockStats(blocks[0]).incomming_paths = 1.0;

        double cost = 0.0;
        foreach (Block b in big_blocks) {
          Contract.Assert(b != null);
          if (keep_at_all.ContainsKey(b)) {
            BlockStats s = GetBlockStats(b);
            UpdateIncommingPaths(s);
            double local = s.assertion_cost;
            if (ShouldAssumize(b)) {
              local = (s.assertion_cost + s.assumption_cost) * CommandLineOptions.Clo.VcsAssumeMult;
            } else {
              local = s.assumption_cost * CommandLineOptions.Clo.VcsAssumeMult + s.assertion_cost;
            }
            local = local + local * s.incomming_paths * CommandLineOptions.Clo.VcsPathCostMult;
            cost += local;
          }
        }

        return cost;
      }

      CmdSeq SliceCmds(Block b) {
        Contract.Requires(b != null);
        Contract.Ensures(Contract.Result<CmdSeq>() != null);

        CmdSeq seq = b.Cmds;
        Contract.Assert(seq != null);
        if (!doing_slice && !ShouldAssumize(b))
          return seq;
        CmdSeq res = new CmdSeq();
        foreach (Cmd c in seq) {
          Contract.Assert(c != null);
          AssertCmd a = c as AssertCmd;
          Cmd the_new = c;
          bool swap = false;
          if (a != null) {
            if (doing_slice) {
              double cost = AssertionCost(a);
              bool first = (slice_limit - cost) >= 0 || slice_initial_limit == slice_limit;
              slice_limit -= cost;
              swap = slice_pos == first;
            } else if (assert_to_assume) {
              swap = true;
            } else {
              Contract.Assert(false);
              throw new cce.UnreachableException();
            }

            if (swap) {
              the_new = AssertTurnedIntoAssume(a);
            }
          }
          res.Add(the_new);
        }
        return res;
      }

      Block CloneBlock(Block b) {
        Contract.Requires(b != null);
        Contract.Ensures(Contract.Result<Block>() != null);

        Block res;
        if (copies.TryGetValue(b, out res)) {
          return cce.NonNull(res);
        }
        res = new Block(b.tok, b.Label, SliceCmds(b), b.TransferCmd);
        GotoCmd gt = b.TransferCmd as GotoCmd;
        copies[b] = res;
        if (gt != null) {
          GotoCmd newGoto = new GotoCmd(gt.tok, new StringSeq(), new BlockSeq());
          res.TransferCmd = newGoto;
          int pos = 0;
          foreach (Block ch in cce.NonNull(gt.labelTargets)) {
            Contract.Assert(ch != null);
            Contract.Assert(doing_slice ||
                   (assert_to_assume || (keep_at_all.ContainsKey(ch) || assumized_branches.Contains(ch))));
            if (doing_slice ||
                ((b != split_block || assumized_branches.Contains(ch) == assert_to_assume) &&
                 keep_at_all.ContainsKey(ch))) {
              newGoto.AddTarget(CloneBlock(ch));
            }
            pos++;
          }
        }
        return res;
      }

      Split DoSplit() {
        Contract.Ensures(Contract.Result<Split>() != null);

        copies.Clear();
        CloneBlock(blocks[0]);
        List<Block> newBlocks = new List<Block>();
        Hashtable newGotoCmdOrigins = new Hashtable();
        foreach (Block b in blocks) {
          Contract.Assert(b != null);
          Block tmp;
          if (copies.TryGetValue(b, out tmp)) {
            newBlocks.Add(cce.NonNull(tmp));
            if (gotoCmdOrigins.ContainsKey(b)) {
              newGotoCmdOrigins[tmp] = gotoCmdOrigins[b];
            }

            foreach (Block p in b.Predecessors) {
              Contract.Assert(p != null);
              Block tmp2;
              if (copies.TryGetValue(p, out tmp2)) {
                tmp.Predecessors.Add(tmp2);
              }
            }
          }
        }

        return new Split(newBlocks, newGotoCmdOrigins, parent, impl);
      }

      Split SplitAt(int idx) {
        Contract.Ensures(Contract.Result<Split>() != null);

        assert_to_assume = idx == 0;
        doing_slice = false;
        ComputeBlockSets(true);

        return DoSplit();
      }

      Split SliceAsserts(double limit, bool pos) {
        Contract.Ensures(Contract.Result<Split>() != null);

        slice_pos = pos;
        slice_limit = limit;
        slice_initial_limit = limit;
        doing_slice = true;
        Split r = DoSplit();
        /*
        Console.WriteLine("split {0} / {1} -->", limit, pos);
        List<Block!> tmp = impl.Blocks;
        impl.Blocks = r.blocks;
        EmitImpl(impl, false);
        impl.Blocks = tmp;
        */

        return r;
      }

      void Print() {
        List<Block> tmp = impl.Blocks;
        Contract.Assert(tmp != null);
        impl.Blocks = blocks;
        EmitImpl(impl, false);
        impl.Blocks = tmp;
      }

      public Counterexample ToCounterexample(ProverContext context) {
        Contract.Requires(context != null);
        Contract.Ensures(Contract.Result<Counterexample>() != null);

        BlockSeq trace = new BlockSeq();
        foreach (Block b in blocks) {
          Contract.Assert(b != null);
          trace.Add(b);
        }
        foreach (Block b in blocks) {
          Contract.Assert(b != null);
          foreach (Cmd c in b.Cmds) {
            Contract.Assert(c != null);
            if (c is AssertCmd) {
              return AssertCmdToCounterexample((AssertCmd)c, cce.NonNull(b.TransferCmd), trace, null, null, context, new Dictionary<Incarnation, Absy>());
            }
          }
        }
        Contract.Assume(false);
        throw new cce.UnreachableException();
      }

      public static List<Split/*!*/>/*!*/ DoSplit(Split initial, double max_cost, int max) {
        Contract.Requires(initial != null);
        Contract.Ensures(cce.NonNullElements(Contract.Result<List<Split>>()));

        List<Split> res = new List<Split>();
        res.Add(initial);

        while (res.Count < max) {
          Split best = null;
          int best_idx = 0, pos = 0;
          foreach (Split s in res) {
            Contract.Assert(s != null);
            s.ComputeBestSplit(); // TODO check total_cost first
            if (s.total_cost > max_cost &&
                (best == null || best.total_cost < s.total_cost) &&
                (s.assertion_count > 1 || s.split_block != null)) {
              best = s;
              best_idx = pos;
            }
            pos++;
          }

          if (best == null)
            break; // no split found

          Split s0, s1;

          bool split_stats = CommandLineOptions.Clo.TraceVerify;

          if (split_stats) {
            Console.WriteLine("{0} {1} -->", best.split_block == null ? "SLICE" : ("SPLIT@" + best.split_block.Label), best.Stats);
            if (best.split_block != null) {
              GotoCmd g = best.split_block.TransferCmd as GotoCmd;
              if (g != null) {
                Console.Write("    exits: ");
                foreach (Block b in cce.NonNull(g.labelTargets)) {
                  Contract.Assert(b != null);
                  Console.Write("{0} ", b.Label);
                }
                Console.WriteLine("");
                Console.Write("    assumized: ");
                foreach (Block b in best.assumized_branches) {
                  Contract.Assert(b != null);
                  Console.Write("{0} ", b.Label);
                }
                Console.WriteLine("");
              }
            }
          }

          if (best.split_block != null) {
            s0 = best.SplitAt(0);
            s1 = best.SplitAt(1);
          } else {
            best.split_block = null;
            s0 = best.SliceAsserts(best.assertion_cost / 2, true);
            s1 = best.SliceAsserts(best.assertion_cost / 2, false);
          }

          if (true) {
            List<Block> ss = new List<Block>();
            ss.Add(s0.blocks[0]);
            ss.Add(s1.blocks[0]);
            try {
              best.SoundnessCheck(new Dictionary<PureCollections.Tuple, bool>(), best.blocks[0], ss);
            } catch (System.Exception e) {
              Console.WriteLine(e);
              best.DumpDot(-1);
              s0.DumpDot(-2);
              s1.DumpDot(-3);
              Contract.Assert(false);
              throw new cce.UnreachableException();
            }
          }

          if (split_stats) {
            s0.ComputeBestSplit();
            s1.ComputeBestSplit();
            Console.WriteLine("    --> {0}", s0.Stats);
            Console.WriteLine("    --> {0}", s1.Stats);
          }

          if (CommandLineOptions.Clo.TraceVerify) {
            best.Print();
          }

          res[best_idx] = s0;
          res.Add(s1);
        }

        return res;
      }

      public Checker Checker {
        get {
          Contract.Ensures(Contract.Result<Checker>() != null);

          Contract.Assert(checker != null);
          return checker;
        }
      }

      public WaitHandle ProverDone {
        get {
          Contract.Assert(checker != null);
          return checker.ProverDone;
        }
      }

      public void ReadOutcome(ref Outcome cur_outcome, out bool prover_failed) {
        Contract.EnsuresOnThrow<UnexpectedProverOutputException>(true);
        ProverInterface.Outcome outcome = cce.NonNull(checker).ReadOutcome();

        if (CommandLineOptions.Clo.Trace && splitNo >= 0) {
          System.Console.WriteLine("      --> split #{0} done,  [{1} s] {2}", splitNo, checker.ProverRunTime.TotalSeconds, outcome);
        }

        if (CommandLineOptions.Clo.VcsDumpSplits) {
          DumpDot(splitNo);
        }

        prover_failed = false;

        switch (outcome) {
          case ProverInterface.Outcome.Valid:
            return;
          case ProverInterface.Outcome.Invalid:
            cur_outcome = Outcome.Errors;
            return;
          case ProverInterface.Outcome.OutOfMemory:
            prover_failed = true;
            if (cur_outcome != Outcome.Errors && cur_outcome != Outcome.Inconclusive)
              cur_outcome = Outcome.OutOfMemory;
            return;
          case ProverInterface.Outcome.TimeOut:
            prover_failed = true;
            if (cur_outcome != Outcome.Errors && cur_outcome != Outcome.Inconclusive)
              cur_outcome = Outcome.TimedOut;
            return;
          case ProverInterface.Outcome.Undetermined:
            if (cur_outcome != Outcome.Errors)
              cur_outcome = Outcome.Inconclusive;
            return;
          default:
            Contract.Assert(false);
            throw new cce.UnreachableException();
        }
      }

      public void BeginCheck(VerifierCallback callback, ModelViewInfo mvInfo, int no, int timeout) {
        Contract.Requires(callback != null);
        splitNo = no;

        impl.Blocks = blocks;

        checker = parent.FindCheckerFor(impl, timeout);
        Hashtable/*<int, Absy!>*/ label2absy = new Hashtable/*<int, Absy!>*/();

        ProverInterface tp = checker.TheoremProver;
        ProverContext ctx = tp.Context;
        Boogie2VCExprTranslator bet = ctx.BoogieExprTranslator;
        bet.SetCodeExprConverter(
          new CodeExprConverter(
          delegate (CodeExpr codeExpr, Hashtable/*<Block, VCExprVar!>*/ blockVariables, List<VCExprLetBinding/*!*/> bindings) {
            VCGen vcgen = new VCGen(new Program(), null, false);
            vcgen.variable2SequenceNumber = new Hashtable/*Variable -> int*/();
            vcgen.incarnationOriginMap = new Dictionary<Incarnation, Absy>();
            vcgen.CurrentLocalVariables = codeExpr.LocVars;
            // codeExpr.Blocks.PruneUnreachableBlocks();  // This is needed for VCVariety.BlockNested, and is otherwise just an optimization

            vcgen.ComputePredecessors(codeExpr.Blocks);
            vcgen.AddBlocksBetween(codeExpr.Blocks);
            Hashtable/*TransferCmd->ReturnCmd*/ gotoCmdOrigins = vcgen.ConvertBlocks2PassiveCmd(codeExpr.Blocks, new IdentifierExprSeq(), new ModelViewInfo(codeExpr));
            VCExpr startCorrect = VCGen.LetVC(
		      codeExpr.Blocks[0],
              null,
              label2absy,
		      blockVariables,
		      bindings,
		      ctx);
            VCExpr vce = ctx.ExprGen.Let(bindings, startCorrect);

            if (vcgen.CurrentLocalVariables.Length != 0) {
              Boogie2VCExprTranslator translator = checker.TheoremProver.Context.BoogieExprTranslator;
              List<VCExprVar> boundVars = new List<VCExprVar>();
              foreach (Variable v in vcgen.CurrentLocalVariables) {
                Contract.Assert(v != null);
                VCExprVar ev = translator.LookupVariable(v);
                Contract.Assert(ev != null);
                boundVars.Add(ev);
                if (v.TypedIdent.Type.Equals(Bpl.Type.Bool)) {
                  // add an antecedent (tickleBool ev) to help the prover find a possible trigger
                  vce = checker.VCExprGen.Implies(checker.VCExprGen.Function(VCExpressionGenerator.TickleBoolOp, ev), vce);
                }
              }
              vce = checker.VCExprGen.Forall(boundVars, new List<VCTrigger>(), vce);
            }
            return vce;
          }
		  ));

        VCExpr vc = parent.GenerateVCAux(impl, null, label2absy, checker);
        Contract.Assert(vc != null);

        if (CommandLineOptions.Clo.vcVariety == CommandLineOptions.VCVariety.Local) {
          reporter = new ErrorReporterLocal(gotoCmdOrigins, label2absy, impl.Blocks, parent.incarnationOriginMap, callback, mvInfo, parent.implName2LazyInliningInfo, cce.NonNull(this.Checker.TheoremProver.Context), parent.program);
        } else {
          reporter = new ErrorReporter(gotoCmdOrigins, label2absy, impl.Blocks, parent.incarnationOriginMap, callback, mvInfo, parent.implName2LazyInliningInfo, this.Checker.TheoremProver.Context, parent.program);
        }

        if (CommandLineOptions.Clo.TraceVerify && no >= 0) {
          Console.WriteLine("-- after split #{0}", no);
          Print();
        }

        string desc = cce.NonNull(impl.Name);
        if (no >= 0)
          desc += "_split" + no;
        checker.BeginCheck(desc, vc, reporter);
      }

      private void SoundnessCheck(Dictionary<PureCollections.Tuple/*!*/, bool>/*!*/ cache, Block/*!*/ orig, List<Block/*!*/>/*!*/ copies) {
        Contract.Requires(cce.NonNullElements(cache));
        Contract.Requires(orig != null);
        Contract.Requires(copies != null);
        {
          PureCollections.Tuple t = new PureCollections.Tuple(new PureCollections.Capacity(1 + copies.Count));
          int i = 0;
          t[i++] = orig;
          foreach (Block b in copies) {
            Contract.Assert(b != null);
            t[i++] = b;
          }
          if (cache.ContainsKey(t)) {
            return;
          }
          cache[t] = true;
        }

        for (int i = 0; i < orig.Cmds.Length; ++i) {
          Cmd cmd = orig.Cmds[i];
          if (cmd is AssertCmd) {
            int found = 0;
            foreach (Block c in copies) {
              Contract.Assert(c != null);
              if (c.Cmds[i] == cmd) {
                found++;
              }
            }
            if (found == 0) {
              throw new System.Exception(string.Format("missing assertion: {0}({1})", cmd.tok.filename, cmd.tok.line));
            }
          }
        }

        foreach (Block exit in Exits(orig)) {
          Contract.Assert(exit != null);
          List<Block> newcopies = new List<Block>();
          foreach (Block c in copies) {
            foreach (Block cexit in Exits(c)) {
              Contract.Assert(cexit != null);
              if (cexit.Label == exit.Label) {
                newcopies.Add(cexit);
              }
            }
          }
          if (newcopies.Count == 0) {
            throw new System.Exception("missing exit " + exit.Label);
          }
          SoundnessCheck(cache, exit, newcopies);
        }
      }
    }
    #endregion


    protected VCExpr GenerateVC(Implementation/*!*/ impl, Variable controlFlowVariable, out Hashtable/*<int, Absy!>*//*!*/ label2absy, Checker/*!*/ ch)
    {
      Contract.Requires(impl != null);
      Contract.Requires(ch != null);
      Contract.Ensures(Contract.ValueAtReturn(out label2absy) != null);
      Contract.Ensures(Contract.Result<VCExpr>() != null);

      label2absy = new Hashtable/*<int, Absy!>*/();
      return GenerateVCAux(impl, controlFlowVariable, label2absy, ch);
    }

    protected VCExpr GenerateVCAux(Implementation/*!*/ impl, Variable controlFlowVariable, Hashtable/*<int, Absy!>*//*!*/ label2absy, Checker/*!*/ ch) {
      Contract.Requires(impl != null);
      Contract.Requires(ch != null);
      Contract.Ensures(Contract.Result<VCExpr>() != null);

      TypecheckingContext tc = new TypecheckingContext(null);
      impl.Typecheck(tc);

      VCExpr vc;
      switch (CommandLineOptions.Clo.vcVariety) {
        case CommandLineOptions.VCVariety.Structured:
          vc = VCViaStructuredProgram(impl, label2absy, ch.TheoremProver.Context);
          break;
        case CommandLineOptions.VCVariety.Block:
          vc = FlatBlockVC(impl, label2absy, false, false, false, ch.TheoremProver.Context);
          break;
        case CommandLineOptions.VCVariety.BlockReach:
          vc = FlatBlockVC(impl, label2absy, false, true, false, ch.TheoremProver.Context);
          break;
        case CommandLineOptions.VCVariety.Local:
          vc = FlatBlockVC(impl, label2absy, true, false, false, ch.TheoremProver.Context);
          break;
        case CommandLineOptions.VCVariety.BlockNested:
          vc = NestedBlockVC(impl, label2absy, false, ch.TheoremProver.Context);
          break;
        case CommandLineOptions.VCVariety.BlockNestedReach:
          vc = NestedBlockVC(impl, label2absy, true, ch.TheoremProver.Context);
          break;
        case CommandLineOptions.VCVariety.Dag:
          if (cce.NonNull(CommandLineOptions.Clo.TheProverFactory).SupportsDags) {
            vc = DagVC(cce.NonNull(impl.Blocks[0]), label2absy, new Hashtable/*<Block, VCExpr!>*/(), ch.TheoremProver.Context);
          } else {
            vc = LetVC(cce.NonNull(impl.Blocks[0]), controlFlowVariable, label2absy, ch.TheoremProver.Context);
          }
          break;
        case CommandLineOptions.VCVariety.Doomed:
          vc = FlatBlockVC(impl, label2absy, false, false, true, ch.TheoremProver.Context);
          break;
        default:
          Contract.Assert(false);
          throw new cce.UnreachableException();  // unexpected enumeration value
      }
      return vc;
    }

    void CheckIntAttributeOnImpl(Implementation impl, string name, ref int val) {
      Contract.Requires(impl != null);
      Contract.Requires(name != null);
      if (!(cce.NonNull(impl.Proc).CheckIntAttribute(name, ref val) || !impl.CheckIntAttribute(name, ref val))) {
        Console.WriteLine("ignoring ill-formed {:{0} ...} attribute on {1}, parameter should be an int", name, impl.Name);
      }
    }

    public override Outcome VerifyImplementation(Implementation/*!*/ impl, Program/*!*/ program, VerifierCallback/*!*/ callback){
      //Contract.Requires(impl != null);
      //Contract.Requires(program != null);
      //Contract.Requires(callback != null);
      Contract.EnsuresOnThrow<UnexpectedProverOutputException>(true);
      if (impl.SkipVerification) {
        return Outcome.Inconclusive; // not sure about this one
      } 
      
      callback.OnProgress("VCgen", 0, 0, 0.0);

      ConvertCFG2DAG(impl, program);

      SmokeTester smoke_tester = null;
      if (CommandLineOptions.Clo.SoundnessSmokeTest) {
        smoke_tester = new SmokeTester(this, impl, program, callback);
        smoke_tester.Copy();
      }

      ModelViewInfo mvInfo;
      Hashtable/*TransferCmd->ReturnCmd*/ gotoCmdOrigins = PassifyImpl(impl, program, out mvInfo);

      double max_vc_cost = CommandLineOptions.Clo.VcsMaxCost;
      int tmp_max_vc_cost = -1, max_splits = CommandLineOptions.Clo.VcsMaxSplits, 
          max_kg_splits = CommandLineOptions.Clo.VcsMaxKeepGoingSplits;
      CheckIntAttributeOnImpl(impl, "vcs_max_cost", ref tmp_max_vc_cost);
      CheckIntAttributeOnImpl(impl, "vcs_max_splits", ref max_splits);
      CheckIntAttributeOnImpl(impl, "vcs_max_keep_going_splits", ref max_kg_splits);
      if (tmp_max_vc_cost >= 0) { 
        max_vc_cost = tmp_max_vc_cost;
      }

      Outcome outcome = Outcome.Correct;

      int cores = CommandLineOptions.Clo.VcsCores;
      Stack<Split> work = new Stack<Split>();
      List<Split> currently_running = new List<Split>();
      ResetPredecessors(impl.Blocks);
      work.Push(new Split(impl.Blocks, gotoCmdOrigins, this, impl));

      bool keep_going = max_kg_splits > 1;
      int total = 0;
      int no = max_splits == 1 && !keep_going ? -1 : 0;
      bool first_round = true;
      bool do_splitting = keep_going || max_splits > 1;
      double remaining_cost = 0.0, proven_cost = 0.0;

      if (do_splitting) {
        remaining_cost = work.Peek().Cost;
      }

      while (work.Count > 0 || currently_running.Count > 0) {
        bool prover_failed = false;
        Split s;

        if (work.Count > 0 && currently_running.Count < cores) {
          s = work.Pop();

          if (first_round && max_splits > 1) {
            prover_failed = true;
            remaining_cost -= s.Cost;
          } else {
            if (CommandLineOptions.Clo.Trace && no >= 0) {
              System.Console.WriteLine("    checking split {1}/{2}, {3:0.00}%, {0} ...", 
                                   s.Stats, no + 1, total, 100 * proven_cost / (proven_cost + remaining_cost));
            }
            callback.OnProgress("VCprove", no < 0 ? 0 : no, total, proven_cost / (remaining_cost + proven_cost));

            s.BeginCheck(callback, mvInfo, no, 
              (keep_going && s.LastChance) ? CommandLineOptions.Clo.VcsFinalAssertTimeout :
                keep_going ? CommandLineOptions.Clo.VcsKeepGoingTimeout :
                             CommandLineOptions.Clo.ProverKillTime);

            no++;

            currently_running.Add(s);
          }
        } else {
          WaitHandle[] handles = new WaitHandle[currently_running.Count];
          for (int i = 0; i < currently_running.Count; ++i) {
            handles[i] = currently_running[i].ProverDone;
          }
          int index = WaitHandle.WaitAny(handles);
          s = currently_running[index];
          currently_running.RemoveAt(index);

          if (do_splitting) {
            remaining_cost -= s.Cost;
          }

          s.ReadOutcome(ref outcome, out prover_failed);

          if (do_splitting) {
            if (prover_failed) {
              // even if the prover fails, we have learned something, i.e., it is 
              // annoying to watch Boogie say Timeout, 0.00% a couple of times
              proven_cost += s.Cost / 100;
            } else {
              proven_cost += s.Cost;
            }
          }
          callback.OnProgress("VCprove", no < 0 ? 0 : no, total, proven_cost / (remaining_cost + proven_cost));

          if (prover_failed && !first_round && s.LastChance) {
            string msg = "some timeout";
            if (s.reporter != null && s.reporter.resourceExceededMessage != null) {
              msg = s.reporter.resourceExceededMessage;
            }
            callback.OnCounterexample(s.ToCounterexample(s.Checker.TheoremProver.Context), msg);
            outcome = Outcome.Errors;
            break;
          }

          Contract.Assert( prover_failed || outcome == Outcome.Correct || outcome == Outcome.Errors || outcome == Outcome.Inconclusive);
        }

        if (prover_failed) { 
          int splits = first_round && max_splits > 1 ? max_splits : max_kg_splits;

          if (splits > 1) {
            List<Split> tmp = Split.DoSplit(s, max_vc_cost, splits);
            Contract.Assert(tmp != null);
            max_vc_cost = 1.0; // for future
            first_round = false;
            //tmp.Sort(new Comparison<Split!>(Split.Compare));
            foreach (Split a in tmp) {
              Contract.Assert(a != null);
              work.Push(a);
              total++;
              remaining_cost += a.Cost;
            }
            if (outcome != Outcome.Errors) {
              outcome = Outcome.Correct;
            }
          } else {
            Contract.Assert( outcome != Outcome.Correct);
            if (outcome == Outcome.TimedOut) {
              string msg = "some timeout";
              if (s.reporter != null && s.reporter.resourceExceededMessage != null) {
                msg = s.reporter.resourceExceededMessage;
              }
              callback.OnTimeout(msg);
            } else if (outcome == Outcome.OutOfMemory) {
              string msg = "out of memory";
              if (s.reporter != null && s.reporter.resourceExceededMessage != null) {
                msg = s.reporter.resourceExceededMessage;
              }
              callback.OnOutOfMemory(msg);
            }

            break;
          }
        }
      }

      if (outcome == Outcome.Correct && smoke_tester != null) {
        smoke_tester.Test();
      }

      callback.OnProgress("done", 0, 0, 1.0);

      return outcome;
    }

    public class ErrorReporter : ProverInterface.ErrorHandler {
      Hashtable/*TransferCmd->ReturnCmd*//*!*/ gotoCmdOrigins;
      Hashtable/*<int, Absy!>*//*!*/ label2absy;
      List<Block/*!*/>/*!*/ blocks;
      protected Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap;
      protected VerifierCallback/*!*/ callback;
      protected ModelViewInfo MvInfo;
      internal string resourceExceededMessage;
      static System.IO.TextWriter modelWriter;
      [ContractInvariantMethod]
      void ObjectInvariant() {
        Contract.Invariant(gotoCmdOrigins != null);
        Contract.Invariant(label2absy != null);
        Contract.Invariant(cce.NonNullElements(blocks));
        Contract.Invariant(cce.NonNullElements(incarnationOriginMap));
        Contract.Invariant(callback != null);
        Contract.Invariant(cce.NonNullElements(implName2LazyInliningInfo));
        Contract.Invariant(context != null);
        Contract.Invariant(program != null);
      }


      public static TextWriter ModelWriter {
        get {
          Contract.Ensures(Contract.Result<TextWriter>() != null);

          if (ErrorReporter.modelWriter == null)
            ErrorReporter.modelWriter = CommandLineOptions.Clo.PrintErrorModelFile == null ? Console.Out : new StreamWriter(CommandLineOptions.Clo.PrintErrorModelFile, false);
          return ErrorReporter.modelWriter;
        }
      }

      Dictionary<string/*!*/, LazyInliningInfo/*!*/>/*!*/ implName2LazyInliningInfo;
      protected ProverContext/*!*/ context;
      Program/*!*/ program;

      public ErrorReporter(Hashtable/*TransferCmd->ReturnCmd*//*!*/ gotoCmdOrigins,
          Hashtable/*<int, Absy!>*//*!*/ label2absy,
          List<Block/*!*/>/*!*/ blocks,
          Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap,
          VerifierCallback/*!*/ callback,
          ModelViewInfo mvInfo,
          Dictionary<string/*!*/, LazyInliningInfo/*!*/>/*!*/ implName2LazyInliningInfo,
          ProverContext/*!*/ context,
          Program/*!*/ program) {
        Contract.Requires(gotoCmdOrigins != null);
        Contract.Requires(label2absy != null);
        Contract.Requires(cce.NonNullElements(blocks));
        Contract.Requires(cce.NonNullElements(incarnationOriginMap));
        Contract.Requires(callback != null);
        Contract.Requires(cce.NonNullElements(implName2LazyInliningInfo));
        Contract.Requires(context!=null);
        Contract.Requires(program!=null);
        this.gotoCmdOrigins = gotoCmdOrigins;
        this.label2absy = label2absy;
        this.blocks = blocks;
        this.incarnationOriginMap = incarnationOriginMap;
        this.callback = callback;
        this.MvInfo = mvInfo;

        this.implName2LazyInliningInfo = implName2LazyInliningInfo;
        this.context = context;
        this.program = program;
        // base();
      }

      public override void OnModel(IList<string/*!*/>/*!*/ labels, ErrorModel errModel) {
        //Contract.Requires(cce.NonNullElements(labels));
        if (CommandLineOptions.Clo.PrintErrorModel >= 1 && errModel != null) {
          if (VC.ConditionGeneration.errorModelList != null)
          {
            VC.ConditionGeneration.errorModelList.Add(errModel);
          }
          errModel.Print(ErrorReporter.ModelWriter);
          ErrorReporter.ModelWriter.Flush();
        }

        Hashtable traceNodes = new Hashtable();
        foreach (string s in labels) {
          Contract.Assert(s != null);
          Absy absy = Label2Absy(s);
          Contract.Assert(absy != null);
          if (traceNodes.ContainsKey(absy))
            System.Console.WriteLine("Warning: duplicate label: " + s + " read while tracing nodes");
          else
            traceNodes.Add(absy, null);
        }

        BlockSeq trace = new BlockSeq();
        Block entryBlock = cce.NonNull(this.blocks[0]);
        Contract.Assert(traceNodes.Contains(entryBlock));
        trace.Add(entryBlock);

        Counterexample newCounterexample = TraceCounterexample(entryBlock, traceNodes, trace, errModel, MvInfo, incarnationOriginMap, implName2LazyInliningInfo, context, program, new Dictionary<TraceLocation, CalleeCounterexampleInfo>());

        if (newCounterexample == null)
          return;

        #region Map passive program errors back to original program errors
        ReturnCounterexample returnExample = newCounterexample as ReturnCounterexample;
        if (returnExample != null) {
          foreach (Block b in returnExample.Trace) {
            Contract.Assert(b != null);
            Contract.Assume(b.TransferCmd != null);
            ReturnCmd cmd = (ReturnCmd)gotoCmdOrigins[b.TransferCmd];
            if (cmd != null) {
              returnExample.FailingReturn = cmd;
              break;
            }
          }
        }
        #endregion
        callback.OnCounterexample(newCounterexample, null);
      }

      public override Absy Label2Absy(string label) {
        //Contract.Requires(label != null);
        Contract.Ensures(Contract.Result<Absy>() != null);

        int id = int.Parse(label);
        return cce.NonNull((Absy)label2absy[id]);
      }

      public override void OnResourceExceeded(string msg) {
        //Contract.Requires(msg != null);
        resourceExceededMessage = msg;
      }

      public override void OnProverWarning(string msg) {
        //Contract.Requires(msg != null);
        callback.OnWarning(msg);
      }
    }

    public class ErrorReporterLocal : ErrorReporter {
      public ErrorReporterLocal(Hashtable/*TransferCmd->ReturnCmd*//*!*/ gotoCmdOrigins,
          Hashtable/*<int, Absy!>*//*!*/ label2absy,
          List<Block/*!*/>/*!*/ blocks,
          Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap,
          VerifierCallback/*!*/ callback,
          ModelViewInfo mvInfo,
          Dictionary<string/*!*/, LazyInliningInfo/*!*/>/*!*/ implName2LazyInliningInfo,
          ProverContext/*!*/ context,
          Program/*!*/ program)
        : base(gotoCmdOrigins, label2absy, blocks, incarnationOriginMap, callback, mvInfo, implName2LazyInliningInfo, context, program) // here for aesthetic purposes //TODO: Maybe nix?
      {
        Contract.Requires(gotoCmdOrigins != null);
        Contract.Requires(label2absy != null);
        Contract.Requires(cce.NonNullElements(blocks));
        Contract.Requires(cce.NonNullElements(incarnationOriginMap));
        Contract.Requires(callback != null);
        Contract.Requires(cce.NonNullElements(implName2LazyInliningInfo));
        Contract.Requires(context != null);
        Contract.Requires(program != null);
      }

      public override void OnModel(IList<string/*!*/>/*!*/ labels, ErrorModel errModel) {
        //Contract.Requires(cce.NonNullElements(labels));
        // We ignore the error model here for enhanced error message purposes.
        // It is only printed to the command line.
        if (CommandLineOptions.Clo.PrintErrorModel >= 1 && errModel != null) {
          if (CommandLineOptions.Clo.PrintErrorModelFile != null) {
            errModel.Print(ErrorReporter.ModelWriter);
            ErrorReporter.ModelWriter.Flush();
          }
        }
        List<Block> traceNodes = new List<Block>();
        List<AssertCmd> assertNodes = new List<AssertCmd>();
        foreach (string s in labels) {
          Contract.Assert(s != null);
          Absy node = Label2Absy(s);
          if (node is Block) {
            Block b = (Block)node;
            traceNodes.Add(b);
          } else {
            AssertCmd a = (AssertCmd)node;
            assertNodes.Add(a);
          }
        }
        Contract.Assert(assertNodes.Count > 0);
        Contract.Assert(traceNodes.Count == assertNodes.Count);

        foreach (AssertCmd a in assertNodes) {
          // find the corresponding Block (assertNodes.Count is likely to be 1, or small in any case, so just do a linear search here)
          foreach (Block b in traceNodes) {
            if (b.Cmds.Has(a)) {
              BlockSeq trace = new BlockSeq();
              trace.Add(b);
              Counterexample newCounterexample = AssertCmdToCounterexample(a, cce.NonNull(b.TransferCmd), trace, errModel, MvInfo, context, incarnationOriginMap);
              callback.OnCounterexample(newCounterexample, null);
              goto NEXT_ASSERT;
            }
          }
          Contract.Assert(false);
          throw new cce.UnreachableException();  // there was no block that contains the assert
        NEXT_ASSERT: {
          }
        }
      }
    }
    protected void ConvertCFG2DAG(Implementation impl, Program program)
    {
    Contract.Requires(impl != null);
    Contract.Requires(program != null);
      impl.PruneUnreachableBlocks();  // This is needed for VCVariety.BlockNested, and is otherwise just an optimization

      CurrentLocalVariables = impl.LocVars;
      variable2SequenceNumber = new Hashtable/*Variable -> int*/();
      incarnationOriginMap = new Dictionary<Incarnation, Absy>();

      #region Debug Tracing
      if (CommandLineOptions.Clo.TraceVerify) 
      {
        Console.WriteLine("original implementation");
        EmitImpl(impl, false);
      }
      #endregion

      #region Debug Tracing
      if (CommandLineOptions.Clo.TraceVerify) 
      {
        Console.WriteLine("after desugaring sugared commands like procedure calls");
        EmitImpl(impl, true);
      }
      #endregion

      ComputePredecessors(impl.Blocks);
      
      #region Convert program CFG into a DAG

      #region Use the graph library to figure out where the (natural) loops are

      #region Create the graph by adding the source node and each edge
      Graph<Block> g = Program.GraphFromImpl(impl);
      #endregion

      g.ComputeLoops(); // this is the call that does all of the processing
      if (!g.Reducible)
      {
        throw new VCGenException("Irreducible flow graphs are unsupported.");
      }

      #endregion

      #region Cut the backedges, push assert/assume statements from loop header into predecessors, change them all into assume statements at top of loop, introduce havoc statements
      foreach (Block header in cce.NonNull( g.Headers))
      {
        Contract.Assert(header != null);
        IDictionary<Block,object> backEdgeNodes = new Dictionary<Block,object>();
        foreach (Block b in cce.NonNull( g.BackEdgeNodes(header))) {Contract.Assert(b != null); backEdgeNodes.Add(b, null); }
      
        #region Find the (possibly empty) prefix of assert commands in the header, replace each assert with an assume of the same condition
        CmdSeq prefixOfPredicateCmdsInit = new CmdSeq();
        CmdSeq prefixOfPredicateCmdsMaintained = new CmdSeq();
        for (int i = 0, n = header.Cmds.Length; i < n; i++)
        {
          PredicateCmd a = header.Cmds[i] as PredicateCmd;
          if (a != null)
          {
            if (a is AssertCmd) {
              Bpl.AssertCmd c = (AssertCmd) a;
              Bpl.AssertCmd b = new Bpl.LoopInitAssertCmd(c.tok, c.Expr);
              b.ErrorData = c.ErrorData;
              prefixOfPredicateCmdsInit.Add(b);
              b = new Bpl.LoopInvMaintainedAssertCmd(c.tok, c.Expr);
              b.ErrorData = c.ErrorData;
              prefixOfPredicateCmdsMaintained.Add(b);
              header.Cmds[i] = new AssumeCmd(c.tok,c.Expr);
            } else {
              Contract.Assert( a is AssumeCmd);
              if (Bpl.CommandLineOptions.Clo.AlwaysAssumeFreeLoopInvariants) {
                // Usually, "free" stuff, like free loop invariants (and the assume statements
                // that stand for such loop invariants) are ignored on the checking side.  This
                // command-line option changes that behavior to always assume the conditions.
                prefixOfPredicateCmdsInit.Add(a);
                prefixOfPredicateCmdsMaintained.Add(a);
              }
            }
          }
          else if ( header.Cmds[i] is CommentCmd )
          {
            // ignore
          }
          else
          {
            break; // stop when an assignment statement (or any other non-predicate cmd) is encountered
          }
        }
        #endregion

        #region Copy the prefix of predicate commands into each predecessor. Do this *before* cutting the backedge!!
        for ( int predIndex = 0, n = header.Predecessors.Length; predIndex < n; predIndex++ )
        {
          Block pred = cce.NonNull(header.Predecessors[predIndex]);
          
          // Create a block between header and pred for the predicate commands if pred has more than one successor 
          GotoCmd gotocmd = cce.NonNull((GotoCmd)pred.TransferCmd);
          Contract.Assert( gotocmd.labelNames != null);  // if "pred" is really a predecessor, it may be a GotoCmd with at least one label
          if (gotocmd.labelNames.Length > 1)
          {
            Block newBlock = CreateBlockBetween(predIndex, header);
            impl.Blocks.Add(newBlock);
            
            // if pred is a back edge node, then now newBlock is the back edge node
            if (backEdgeNodes.ContainsKey(pred))
            {
              backEdgeNodes.Remove(pred);
              backEdgeNodes.Add(newBlock,null);
            }
            
            pred = newBlock;
          } 
          // Add the predicate commands
          if (backEdgeNodes.ContainsKey(pred)){
            pred.Cmds.AddRange(prefixOfPredicateCmdsMaintained);
          }
          else {
            pred.Cmds.AddRange(prefixOfPredicateCmdsInit);
          }
        }
        #endregion

        #region Cut the back edge
        foreach (Block backEdgeNode in cce.NonNull(backEdgeNodes.Keys))
        {Contract.Assert(backEdgeNode != null);
          Debug.Assert(backEdgeNode.TransferCmd is GotoCmd,"An node was identified as the source for a backedge, but it does not have a goto command.");
          GotoCmd gtc = backEdgeNode.TransferCmd as GotoCmd;
          if (gtc != null && gtc.labelTargets != null && gtc.labelTargets.Length > 1 )
          {
            // then remove the backedge by removing the target block from the list of gotos
            BlockSeq remainingTargets = new BlockSeq();
            StringSeq remainingLabels = new StringSeq();
            Contract.Assume( gtc.labelNames != null);
            for (int i = 0, n = gtc.labelTargets.Length; i < n; i++)
            {
              if ( gtc.labelTargets[i] != header )
              {
                remainingTargets.Add(gtc.labelTargets[i]);
                remainingLabels.Add(gtc.labelNames[i]);
              }
            }
            gtc.labelTargets = remainingTargets;
            gtc.labelNames = remainingLabels;
          }
          else
          {
            // This backedge is the only out-going edge from this node.
            // Add an "assume false" statement to the end of the statements
            // inside of the block and change the goto command to a return command.
            AssumeCmd ac = new AssumeCmd(Token.NoToken,Expr.False);
            backEdgeNode.Cmds.Add(ac);
            backEdgeNode.TransferCmd = new ReturnCmd(Token.NoToken);
          }
          #region Remove the backedge node from the list of predecessor nodes in the header
          BlockSeq newPreds = new BlockSeq();
          foreach ( Block p in header.Predecessors )
          {
            if ( p != backEdgeNode )
              newPreds.Add(p);
          }
          header.Predecessors = newPreds;
          #endregion
        }
        #endregion

        #region Collect all variables that are assigned to in all of the natural loops for which this is the header
        VariableSeq varsToHavoc = new VariableSeq();
        foreach (Block backEdgeNode in cce.NonNull( g.BackEdgeNodes(header)))
        {
          Contract.Assert(backEdgeNode != null);
          foreach ( Block b in g.NaturalLoops(header,backEdgeNode) )
          {
            Contract.Assert(b != null);
            foreach ( Cmd c in b.Cmds )
            {
              Contract.Assert(c != null);
              c.AddAssignedVariables(varsToHavoc);
            }
          }
        }
        IdentifierExprSeq havocExprs = new IdentifierExprSeq();
        foreach ( Variable v in varsToHavoc )
        {
          Contract.Assert(v != null);
          IdentifierExpr ie = new IdentifierExpr(Token.NoToken, v);
          if(!havocExprs.Has(ie))
            havocExprs.Add(ie);
        }
        // pass the token of the enclosing loop header to the HavocCmd so we can reconstruct
        // the source location for this later on
        HavocCmd hc = new HavocCmd(header.tok,havocExprs);
        CmdSeq newCmds = new CmdSeq();
        newCmds.Add(hc);
        foreach ( Cmd c in header.Cmds )
        {
          newCmds.Add(c);
        }
        header.Cmds = newCmds;
        #endregion
      }
      #endregion
      #endregion Convert program CFG into a DAG
      
      #region Debug Tracing
      if (CommandLineOptions.Clo.TraceVerify) 
      {
        Console.WriteLine("after conversion into a DAG");
        EmitImpl(impl, true);
      }
      #endregion
    }

    protected Hashtable/*TransferCmd->ReturnCmd*/ PassifyImpl(Implementation impl, Program program, out ModelViewInfo mvInfo)
    {
      Contract.Requires(impl != null);
      Contract.Requires(program != null);
      Contract.Ensures(Contract.Result<Hashtable>() != null);

      Hashtable/*TransferCmd->ReturnCmd*/ gotoCmdOrigins = new Hashtable/*TransferCmd->ReturnCmd*/();
      Block exitBlock = GenerateUnifiedExit(impl, gotoCmdOrigins);
      
      #region Debug Tracing
      if (CommandLineOptions.Clo.TraceVerify) 
      {
        Console.WriteLine("after creating a unified exit block");
        EmitImpl(impl, true);
      }
      #endregion

      #region Insert pre- and post-conditions and where clauses as assume and assert statements
      {
        CmdSeq cc = new CmdSeq();
        // where clauses of global variables
        foreach (Declaration d in program.TopLevelDeclarations) {
          GlobalVariable gvar = d as GlobalVariable;
          if (gvar != null && gvar.TypedIdent.WhereExpr != null) {
            Cmd c = new AssumeCmd(gvar.tok, gvar.TypedIdent.WhereExpr);
            cc.Add(c);
          }
        }
        // where clauses of in- and out-parameters
        cc.AddRange(GetParamWhereClauses(impl));
        // where clauses of local variables
        foreach (Variable lvar in impl.LocVars) {Contract.Assert(lvar != null);
          if (lvar.TypedIdent.WhereExpr != null) {
            Cmd c = new AssumeCmd(lvar.tok, lvar.TypedIdent.WhereExpr);
            cc.Add(c);
          }
        }
        // add cc and the preconditions to new blocks preceding impl.Blocks[0]
        InjectPreconditions(impl, cc);

        // append postconditions, starting in exitBlock and continuing into other blocks, if needed
        exitBlock = InjectPostConditions(impl, exitBlock, gotoCmdOrigins);
      }
      #endregion
      
      #region Support for lazy/stratified inlining
      addExitAssert(impl.Name, exitBlock);
      #endregion

                 
      #region Debug Tracing
      if (CommandLineOptions.Clo.TraceVerify) 
      {
        Console.WriteLine("after inserting pre- and post-conditions");
        EmitImpl(impl, true);
      }
      #endregion

      AddBlocksBetween(impl.Blocks);
      
      #region Debug Tracing
      if (CommandLineOptions.Clo.TraceVerify) 
      {
        Console.WriteLine("after adding empty blocks as needed to catch join assumptions");
        EmitImpl(impl, true);
      }
      #endregion
    
      if (CommandLineOptions.Clo.LiveVariableAnalysis > 0) {
        Microsoft.Boogie.LiveVariableAnalysis.ComputeLiveVariables(impl);
      }

      mvInfo = new ModelViewInfo(program, impl);
      Hashtable exitIncarnationMap = Convert2PassiveCmd(impl, mvInfo);

      #region Support for lazy/stratified inlining
      storeIncarnationMaps(impl.Name, exitIncarnationMap);
      #endregion
            
      if (CommandLineOptions.Clo.LiveVariableAnalysis == 1) {
        Microsoft.Boogie.LiveVariableAnalysis.ClearLiveVariables(impl);
      } 
      // TODO: fix
      //else if (CommandLineOptions.Clo.LiveVariableAnalysis == 2) {
      //  Microsoft.Boogie.InterProcGenKill.ClearLiveVariables(impl, program);
      //}
      
      #region Peep-hole optimizations
      if (CommandLineOptions.Clo.RemoveEmptyBlocks){
        #region Get rid of empty blocks
        {
          Block entryBlock = cce.NonNull( impl.Blocks[0]);
          RemoveEmptyBlocks(entryBlock);
          impl.PruneUnreachableBlocks();
        }
        #endregion Get rid of empty blocks
        
        #region Debug Tracing
        if (CommandLineOptions.Clo.TraceVerify) 
        {
          Console.WriteLine("after peep-hole optimizations");
          EmitImpl(impl, true);
        }
        #endregion
      }
      #endregion Peep-hole optimizations

      if (CommandLineOptions.Clo.ExpandLambdas)
      {
        List<Expr> axioms;
        List<Function> functions;
        LambdaHelper.Desugar(impl, out axioms, out functions);
        program.TopLevelDeclarations.AddRange(functions);

        if (axioms.Count > 0) {
          CmdSeq cmds = new CmdSeq();
          foreach (Expr ax in axioms) {
            Contract.Assert(ax != null);
            cmds.Add(new AssumeCmd(ax.tok, ax));
          }
          Block entryBlock = cce.NonNull( impl.Blocks[0]);
          cmds.AddRange(entryBlock.Cmds);
          entryBlock.Cmds = cmds;
        }
      }

      HandleSelectiveChecking(impl);


//      #region Constant Folding
//      #endregion
//      #region Debug Tracing
//      if (CommandLineOptions.Clo.TraceVerify) 
//      {
//        Console.WriteLine("after constant folding");
//        EmitImpl(impl, true);
//      }
//      #endregion

      return gotoCmdOrigins;
    }

    private static void HandleSelectiveChecking(Implementation impl)
    {
      if (QKeyValue.FindBoolAttribute(impl.Attributes, "selective_checking") ||
          QKeyValue.FindBoolAttribute(impl.Proc.Attributes, "selective_checking")) {

        var startPoints = new List<Block>();
        foreach (var b in impl.Blocks) {
          foreach (Cmd c in b.Cmds) {
            var p = c as PredicateCmd;
            if (p != null && QKeyValue.FindBoolAttribute(p.Attributes, "start_checking_here")) {
              startPoints.Add(b);
              break;
            }
          }
        }

        var blocksToCheck = new HashSet<Block>();
        foreach (var b in startPoints) {
          var todo = new Stack<Block>();
          var wasThere = blocksToCheck.Contains(b);
          todo.Push(b);
          while (todo.Count > 0) {
            var x = todo.Pop();
            if (blocksToCheck.Contains(x)) continue;
            blocksToCheck.Add(x);
            var ex = x.TransferCmd as GotoCmd;
            if (ex != null)
              foreach (Block e in ex.labelTargets)
                todo.Push(e);
          }
          if (!wasThere) blocksToCheck.Remove(b);
        }

        foreach (var b in impl.Blocks) {
          if (blocksToCheck.Contains(b)) continue;
          var newCmds = new CmdSeq();
          var copyMode = false;
          foreach (Cmd c in b.Cmds) {
            var p = c as PredicateCmd;
            if (p != null && QKeyValue.FindBoolAttribute(p.Attributes, "start_checking_here"))
              copyMode = true;
            var asrt = c as AssertCmd;
            if (copyMode || asrt == null)
              newCmds.Add(c);
            else
              newCmds.Add(AssertTurnedIntoAssume(asrt));
          }

          b.Cmds = newCmds;
        }
      }
    }

    // Used by lazy/stratified inlining
    protected virtual void addExitAssert(string implName, Block exitBlock)
    {
        if (implName2LazyInliningInfo != null && implName2LazyInliningInfo.ContainsKey(implName))
        {
            Expr assertExpr = implName2LazyInliningInfo[implName].assertExpr;
            Contract.Assert(assertExpr != null);
            exitBlock.Cmds.Add(new AssertCmd(Token.NoToken, assertExpr));
        }
    }

    protected virtual void storeIncarnationMaps(string implName, Hashtable exitIncarnationMap)
    {
        if (implName2LazyInliningInfo != null && implName2LazyInliningInfo.ContainsKey(implName))
        {
            LazyInliningInfo info = implName2LazyInliningInfo[implName];
            Contract.Assert(info != null);
            info.exitIncarnationMap = exitIncarnationMap;
            info.incarnationOriginMap = this.incarnationOriginMap;
        }
    }

    public Counterexample extractLoopTrace(Counterexample cex, string mainProcName, Program program, Dictionary<string, Dictionary<string, Block>> extractLoopMappingInfo)
    {
        // Construct the set of inlined procs in the original program
        var inlinedProcs = new Dictionary<string, Procedure>();
        foreach (var decl in program.TopLevelDeclarations)
        {
            if (decl is Procedure)
            {
                if (!(decl is LoopProcedure))
                {
                    var p = decl as Procedure;
                    inlinedProcs.Add(p.Name, p);
                }
            }
        }

        return extractLoopTraceRec(
            new CalleeCounterexampleInfo(cex, new List<object>()),
            mainProcName, inlinedProcs, extractLoopMappingInfo).counterexample;
    }

    private CalleeCounterexampleInfo extractLoopTraceRec(
        CalleeCounterexampleInfo cexInfo, string currProc,
        Dictionary<string, Procedure> inlinedProcs, 
        Dictionary<string, Dictionary<string, Block>> extractLoopMappingInfo)
    {
        Contract.Requires(currProc != null);

        var cex = cexInfo.counterexample;
        // Go through all blocks in the trace, map them back to blocks in the original program (if there is one)
        var ret = cex.Clone();
        ret.Trace = new BlockSeq();
        ret.calleeCounterexamples = new Dictionary<TraceLocation, CalleeCounterexampleInfo>();

        for (int numBlock = 0; numBlock < cex.Trace.Length; numBlock ++ )
        {
            Block block = cex.Trace[numBlock];
            var origBlock = elGetBlock(currProc, block, extractLoopMappingInfo);
            if (origBlock != null) ret.Trace.Add(origBlock);
            var callCnt = 1;
            for (int numInstr = 0; numInstr < block.Cmds.Length; numInstr ++) {
                Cmd cmd = block.Cmds[numInstr];
                var loc = new TraceLocation(numBlock, numInstr);
                if (!cex.calleeCounterexamples.ContainsKey(loc)) continue;
                string callee = cex.getCalledProcName(cex.getTraceCmd(loc));
                Contract.Assert(callee != null);
                var calleeTrace = cex.calleeCounterexamples[loc];
                Debug.Assert(calleeTrace != null);

                var origTrace = extractLoopTraceRec(calleeTrace, callee, inlinedProcs, extractLoopMappingInfo);

                if (elIsLoop(callee))
                {
                    // Absorb the trace into the current trace

                    int currLen = ret.Trace.Length;
                    ret.Trace.AddRange(origTrace.counterexample.Trace);

                    foreach (var kvp in origTrace.counterexample.calleeCounterexamples)
                    {
                        var newloc = new TraceLocation(kvp.Key.numBlock + currLen, kvp.Key.numInstr);
                        ret.calleeCounterexamples.Add(newloc, kvp.Value);
                    }

                }
                else
                {
                    var origLoc = new TraceLocation(ret.Trace.Length - 1, getCallCmdPosition(origBlock, callCnt, inlinedProcs, callee));
                    ret.calleeCounterexamples.Add(origLoc, origTrace);
                    callCnt++;
                }
            }
        }
        return new CalleeCounterexampleInfo(ret, cexInfo.args);
    }

    // return the position of the i^th CallCmd in the block (count only those Calls that call a procedure in inlinedProcs). 
    // Assert failure if there isn't any.
    // Assert that the CallCmd found calls "callee"
    private int getCallCmdPosition(Block block, int i, Dictionary<string, Procedure> inlinedProcs, string callee)
    {
        Debug.Assert(i >= 1);
        for (int pos = 0; pos < block.Cmds.Length; pos++)
        {
            Cmd cmd = block.Cmds[pos];
            string procCalled = null;
            if (cmd is CallCmd)
            {
                var cc = (CallCmd)cmd;
                if (inlinedProcs.ContainsKey(cc.Proc.Name))
                {
                    procCalled = cc.Proc.Name;
                }
            }

            if (cmd is AssumeCmd)
            {
                var expr = (cmd as AssumeCmd).Expr as NAryExpr;
                if (expr != null)
                {
                    if (inlinedProcs.ContainsKey(expr.Fun.FunctionName))
                    {
                        procCalled = expr.Fun.FunctionName;
                    }
                }
            }

            if (procCalled != null)
            {
                if (i == 1)
                {
                    Debug.Assert(procCalled == callee);
                    return pos;
                }
                i--;
            }
        }

        Debug.Assert(false, "Didn't find the i^th call cmd");
        return -1;
    }

    protected virtual bool elIsLoop(string procname)
    {
        Contract.Requires(procname != null);

        LazyInliningInfo info = null;
        if (implName2LazyInliningInfo.ContainsKey(procname))
        {
            info = implName2LazyInliningInfo[procname];
        }

        if (info == null) return false;

        var lp = info.impl.Proc as LoopProcedure;

        if (lp == null) return false;
        return true;
    }

    private Block elGetBlock(string procname, Block block, Dictionary<string, Dictionary<string, Block>> extractLoopMappingInfo)
    {
        Contract.Requires(procname != null);

        if (!extractLoopMappingInfo.ContainsKey(procname))
            return block;

        if (!extractLoopMappingInfo[procname].ContainsKey(block.Label))
            return null;

        return extractLoopMappingInfo[procname][block.Label];
    }

    private static Counterexample LazyCounterexample(
                                       ErrorModel/*!*/ errModel, ModelViewInfo mvInfo,
                                       Dictionary<string/*!*/, LazyInliningInfo/*!*/>/*!*/ implName2LazyInliningInfo,
                                       ProverContext/*!*/ context,
                                       Program/*!*/ program,
                                       string/*!*/ implName, List<int>/*!*/ values)
    {
      Contract.Requires(errModel != null);
      Contract.Requires(cce.NonNullElements(implName2LazyInliningInfo));
      Contract.Requires(context != null);
      Contract.Requires(program != null);
      Contract.Requires(implName != null);
      Contract.Requires(values != null);
      Contract.Ensures(Contract.Result<Counterexample>() != null);

      Boogie2VCExprTranslator boogieExprTranslator = context.BoogieExprTranslator;
      Contract.Assert(boogieExprTranslator != null);
      LazyInliningInfo info = implName2LazyInliningInfo[implName];
      Contract.Assert(info != null);
      BlockSeq trace = new BlockSeq();
      Block b = cce.NonNull( info.impl).Blocks[0];
      trace.Add(b);
      VCExprVar cfcVar = boogieExprTranslator.LookupVariable(info.controlFlowVariable);
      string cfcName = context.Lookup(cfcVar);
      int cfcPartition = errModel.LookupSkolemFunctionAt(cfcName + "!" + info.uniqueId, values);
      int cfcValue = errModel.LookupPartitionValue(cfcPartition);
      
      var calleeCounterexamples = new Dictionary<TraceLocation, CalleeCounterexampleInfo>();
      while (true) {
        CmdSeq cmds = b.Cmds;Contract.Assert(cmds != null);
        TransferCmd transferCmd = cce.NonNull(b.TransferCmd);
        for (int i = 0; i < cmds.Length; i++)
        {
          Cmd cmd = cce.NonNull( cmds[i]);
          AssertCmd assertCmd = cmd as AssertCmd;
          if (assertCmd != null && errModel.LookupControlFlowFunctionAt(cfcValue, assertCmd.UniqueId) == 0)
          {
            Counterexample newCounterexample;
            newCounterexample = AssertCmdToCounterexample(assertCmd, transferCmd, trace, errModel, mvInfo, context, cce.NonNull(info.incarnationOriginMap));
            newCounterexample.AddCalleeCounterexample(calleeCounterexamples);
            return newCounterexample;
          }
            
          AssumeCmd assumeCmd = cmd as AssumeCmd;
          if (assumeCmd == null) continue;
          NAryExpr naryExpr = assumeCmd.Expr as NAryExpr;
          if (naryExpr == null) continue;
          string calleeName = naryExpr.Fun.FunctionName;
          Contract.Assert(calleeName != null);
          if (!implName2LazyInliningInfo.ContainsKey(calleeName)) continue;
      
          List<int> args = new List<int>();
          foreach (Expr expr in naryExpr.Args)
          {Contract.Assert(expr != null);
            VCExprVar exprVar;
            string name;
            LiteralExpr litExpr = expr as LiteralExpr;
            if (litExpr != null) 
            {
              args.Add(errModel.valueToPartition[litExpr.Val]);
              continue;
            }
          
            IdentifierExpr idExpr = expr as IdentifierExpr;
            Contract.Assert( idExpr != null);
            Variable var = cce.NonNull(idExpr.Decl);
            
            if (var is Constant) 
            {
              exprVar = boogieExprTranslator.LookupVariable(var);
              name = context.Lookup(exprVar);
              args.Add(errModel.identifierToPartition[name]);
              continue;
            }
            
            int index = 0;
            List<GlobalVariable> globalVars = program.GlobalVariables();
            foreach (Variable global in globalVars)
            {
              Contract.Assert(global != null);
              if (global == var) break;
              index++;
            }
            if (index < globalVars.Count)
            {
              args.Add(values[index]);
              continue;
            }
            
            foreach (Variable input in info.impl.InParams)
            {
              Contract.Assert(input != null);
              if (input == var) break;
              index++;
            }
            if (index < globalVars.Count + info.impl.InParams.Length)
            {
              args.Add(values[index]);
              continue;
            }
              
            foreach (Variable output in info.impl.OutParams)
            {
              Contract.Assert(output != null);
              if (output == var) break;
              index++;
            }
            if (index < globalVars.Count + info.impl.InParams.Length + info.impl.OutParams.Length)
            {
              args.Add(values[index]);
              continue;
            }
            
            exprVar = boogieExprTranslator.LookupVariable(var);
            name = context.Lookup(exprVar);
            args.Add(errModel.LookupSkolemFunctionAt(name + "!" + info.uniqueId, values));
          }
          calleeCounterexamples[new TraceLocation(trace.Length - 1, i)] = 
            new CalleeCounterexampleInfo(
              LazyCounterexample(errModel, mvInfo, implName2LazyInliningInfo, context, program, calleeName, args),
              errModel.PartitionsToValues(args));
        }
          
        GotoCmd gotoCmd = transferCmd as GotoCmd;
        if (gotoCmd == null) break;
        int nextBlockId = errModel.LookupControlFlowFunctionAt(cfcValue, b.UniqueId);
        b = (Block)cce.NonNull(info.label2absy)[nextBlockId];
        trace.Add(b);
      }
      Contract.Assert(false);throw new cce.UnreachableException();
    }

    static Counterexample TraceCounterexample(Block b, BlockSeq trace, ErrorModel errModel, Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap) {
      Contract.Requires(b != null);
      Contract.Requires(trace != null);
      Contract.Requires(errModel != null);
      Contract.Requires(cce.NonNullElements(incarnationOriginMap));
      // After translation, all potential errors come from asserts.

      return null;
    }

    static Counterexample TraceCounterexample(
                          Block/*!*/ b, Hashtable/*!*/ traceNodes, BlockSeq/*!*/ trace, ErrorModel errModel, ModelViewInfo mvInfo,
                          Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap,
                          Dictionary<string/*!*/, LazyInliningInfo/*!*/>/*!*/ implName2LazyInliningInfo,
                          ProverContext/*!*/ context, Program/*!*/ program,
                          Dictionary<TraceLocation/*!*/, CalleeCounterexampleInfo/*!*/>/*!*/ calleeCounterexamples)
    {
      Contract.Requires(b != null);
      Contract.Requires(traceNodes != null);
      Contract.Requires(trace != null);
      Contract.Requires(errModel != null);
      Contract.Requires(cce.NonNullElements(incarnationOriginMap));
      Contract.Requires(cce.NonNullElements(implName2LazyInliningInfo));
      Contract.Requires(context != null);
      Contract.Requires(program != null);
      Contract.Requires(cce.NonNullElements(calleeCounterexamples));
      // After translation, all potential errors come from asserts.
      CmdSeq cmds = b.Cmds;
      Contract.Assert(cmds != null);
      TransferCmd transferCmd = cce.NonNull(b.TransferCmd);
      for (int i = 0; i < cmds.Length; i++)
      {
        Cmd cmd = cce.NonNull( cmds[i]);
            
        // Skip if 'cmd' not contained in the trace or not an assert
        if (cmd is AssertCmd && traceNodes.Contains(cmd))
        {
          Counterexample newCounterexample = AssertCmdToCounterexample((AssertCmd)cmd, transferCmd, trace, errModel, mvInfo, context, incarnationOriginMap);
          Contract.Assert(newCounterexample != null);
          newCounterexample.AddCalleeCounterexample(calleeCounterexamples);
          return newCounterexample;
        }
        
        #region Counterexample generation for lazily inlined procedures
        if (errModel == null) continue;
        AssumeCmd assumeCmd = cmd as AssumeCmd;
        if (assumeCmd == null) continue;
        NAryExpr naryExpr = assumeCmd.Expr as NAryExpr;
        if (naryExpr == null) continue;
        string calleeName = naryExpr.Fun.FunctionName;
        if (!implName2LazyInliningInfo.ContainsKey(calleeName)) continue;
        Boogie2VCExprTranslator boogieExprTranslator = context.BoogieExprTranslator;
        Contract.Assert(boogieExprTranslator != null);
        List<int> args = new List<int>();
        foreach (Expr expr in naryExpr.Args)
        {Contract.Assert(expr != null);
          LiteralExpr litExpr = expr as LiteralExpr;
          if (litExpr != null) 
          {
            args.Add(errModel.valueToPartition[litExpr.Val]);
            continue;
          }
          
          IdentifierExpr idExpr = expr as IdentifierExpr;
          Contract.Assert( idExpr != null);
          Contract.Assert( idExpr.Decl != null);
          VCExprVar var = boogieExprTranslator.LookupVariable(idExpr.Decl);
          Contract.Assert(var != null);
          string name = context.Lookup(var);
          Contract.Assert(name != null);
          args.Add(errModel.identifierToPartition[name]);
        }
        calleeCounterexamples[new TraceLocation(trace.Length - 1, i)] = 
            new CalleeCounterexampleInfo(
                LazyCounterexample(errModel, mvInfo, implName2LazyInliningInfo, context, program, calleeName, args),
                errModel.PartitionsToValues(args));
        #endregion
      }
      
      GotoCmd gotoCmd = transferCmd as GotoCmd;
      if (gotoCmd != null)
      {
        foreach (Block bb in cce.NonNull(gotoCmd.labelTargets))
        {
          Contract.Assert(bb != null);
          if (traceNodes.Contains(bb)){
            trace.Add(bb);
            return TraceCounterexample(bb, traceNodes, trace, errModel, mvInfo, incarnationOriginMap, implName2LazyInliningInfo, context, program, calleeCounterexamples);
          }
        }
      }

      return null;

      // Debug.Fail("Could not find failing node.");
      // throw new Microsoft.Contracts.AssertException();
    }


    static void /*return printable error!*/ ApplyEnhancedErrorPrintingStrategy(Bpl.Expr/*!*/ expr, Hashtable /*Variable -> Expr*//*!*/ incarnationMap,
      MiningStrategy errorDataEnhanced, ErrorModel/*!*/ errModel, Dictionary<Expr/*!*/, object>/*!*/ exprToPrintableValue,
      List<string/*!*/>/*!*/ relatedInformation, bool printInternalStateDumpOnce, Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap) {
      Contract.Requires(expr != null);
      Contract.Requires(incarnationMap != null);
      Contract.Requires(errModel != null);
      Contract.Requires(cce.NonNullElements(exprToPrintableValue));
      Contract.Requires(cce.NonNullElements(relatedInformation));
      Contract.Requires(cce.NonNullElements(incarnationOriginMap));
      if (errorDataEnhanced is ListOfMiningStrategies) {
        ListOfMiningStrategies loms = (ListOfMiningStrategies)errorDataEnhanced;
        List < MiningStrategy > l = loms.msList;
        for (int i = 0; i < l.Count; i++) {
          MiningStrategy ms = l[i];
          if (ms != null) {
            ApplyEnhancedErrorPrintingStrategy(expr, incarnationMap, l[i], errModel, exprToPrintableValue, relatedInformation, false, incarnationOriginMap);
          }
        }
      } else if (errorDataEnhanced is EEDTemplate /*EDEverySubExpr*/) {
        EEDTemplate eedT = (EEDTemplate)errorDataEnhanced;
        string reason = eedT.reason;
        List<Bpl.Expr> listOfExprs = eedT.exprList;
        Contract.Assert(cce.NonNullElements(listOfExprs));
        if (listOfExprs != null) {
          List<string> holeFillers = new List<string>();
          for (int i = 0; i < listOfExprs.Count; i++) {
            bool alreadySet = false;
            foreach (KeyValuePair<Bpl.Expr, object> kvp in exprToPrintableValue) {
              Contract.Assert(kvp.Key != null);
              Bpl.Expr e = kvp.Key;
              Bpl.Expr f = listOfExprs[i];
              // the strings are compared instead of the actual expressions because 
              // the expressions might not be identical, but their print-out strings will be
              if (e.ToString() == f.ToString()) {
                object o = kvp.Value;
                if (o != null) {
                  holeFillers.Add(o.ToString());
                  alreadySet = true;
                  break;
                }
              }
            }
            if (!alreadySet) {
              // no information about that Expression found, so put in <unknown>
              holeFillers.Add("<unknown>");
            }
          }
          reason = FormatReasonString(reason, holeFillers);
        }
        if (reason != null) {
          relatedInformation.Add("(related information): " + reason);
        }
      } else {
        // define new templates here!
      }

      if (printInternalStateDumpOnce) {
        ComputeAndTreatHeapSuccessions(incarnationMap, errModel, incarnationOriginMap, relatedInformation);

        // default action: print all values!
        foreach (KeyValuePair<Bpl.Expr, object> kvp in exprToPrintableValue) {
          Contract.Assert(kvp.Key != null);
          object o = kvp.Value;
          if (o != null) {
            // We do not want to print LiteralExprs because that gives things like 0 == 0.
            // If both arguments to the string.Format are the same it is also useless, 
            //   as that would print e.g. $a == $a.
            if (!(kvp.Key is LiteralExpr) && kvp.Key.ToString() != o.ToString()) {
              string boogieExpr;
              // check whether we are handling BPL or SSC input
              if (CommandLineOptions.Clo.RunningBoogieOnSsc) {
                boogieExpr = Helpers.PrettyPrintBplExpr(kvp.Key);
              } else {
                boogieExpr = kvp.Key.ToString();
              }
              relatedInformation.Add("(internal state dump): " + string.Format("{0} == {1}", boogieExpr, o));
            }
          }
        }
      }
    }

    static void ComputeAndTreatHeapSuccessions(System.Collections.Hashtable/*!*/ incarnationMap, ErrorModel/*!*/ errModel,
      Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap, List<string/*!*/>/*!*/ relatedInformation) {
      Contract.Requires(incarnationMap != null);
      Contract.Requires(errModel != null);
      Contract.Requires(cce.NonNullElements(incarnationOriginMap));
      Contract.Requires(cce.NonNullElements(relatedInformation));
      List<int> heapSuccList = ComputeHeapSuccessions(incarnationMap, errModel);
      TreatHeapSuccessions(heapSuccList, incarnationMap, errModel, incarnationOriginMap, relatedInformation);
    }

    static List<int> ComputeHeapSuccessions(System.Collections.Hashtable incarnationMap, ErrorModel errModel) {
      Contract.Requires(incarnationMap != null);
      Contract.Requires(errModel != null);
      // find the heap variable
      Variable heap = null;
      ICollection ic = incarnationMap.Keys;
      foreach (object o in ic) {
        if (o is GlobalVariable) {
          GlobalVariable gv = (GlobalVariable)o;
          if (gv.Name == "$Heap") {
            heap = gv;
          }
        }
      }
      List<int> heapIdSuccession = new List<int>();
      if (heap == null) {
        // without knowing the name of the current heap we cannot create a heap succession!
      } else {
        object oHeap = incarnationMap[heap];
        if (oHeap != null) {
          string currentHeap = oHeap.ToString();
          int currentHeapId;
          if (errModel.identifierToPartition.TryGetValue(currentHeap, out currentHeapId)) {
            while (currentHeapId != -1) {
              if (!heapIdSuccession.Contains(currentHeapId)) {
                heapIdSuccession.Add(currentHeapId);
                currentHeapId = ComputePredecessorHeapId(currentHeapId, errModel);
              } else {
                // looping behavior, just stop here and do not add this value (again!)
                break;
              }
            }
          }
        }
      }
      if (heapIdSuccession.Count > 0) {
        int heapId = heapIdSuccession[heapIdSuccession.Count - 1];
        List<string> strl = errModel.partitionToIdentifiers[heapId];
        Contract.Assert(strl != null);
        if (strl != null && strl.Contains("$Heap")) {
          // we have a proper succession of heaps that starts with $Heap
          return heapIdSuccession;
        } else {
          // no proper heap succession, not starting with $Heap!
          return null;
        }
      } else {
        // no heap succession found because either the $Heap does not have a current incarnation
        // or because (unlikely!) the model is somehow messed up
        return null;
      }
    }

    static int ComputePredecessorHeapId(int id, ErrorModel errModel) {
      Contract.Requires(errModel != null);
      //check "$HeapSucc" and "store2" functions:
      List<int> heapSuccPredIdList = new List<int>();
      List<List<int>> heapSuccFunc;
      errModel.definedFunctions.TryGetValue("$HeapSucc", out heapSuccFunc);
      if (heapSuccFunc != null) {
        foreach (List<int> heapSuccFuncDef in heapSuccFunc) {
          // do not look at the else case of the function def, so check .Count
          if (heapSuccFuncDef != null && heapSuccFuncDef.Count == 3 && heapSuccFuncDef[1] == id) {
            // make sure each predecessor is put into the list at most once
            if (!heapSuccPredIdList.Contains(heapSuccFuncDef[0])) {
              heapSuccPredIdList.Add(heapSuccFuncDef[0]);
            }
          }
        }
      }
      List<int> store2PredIdList = new List<int>();
      ;
      List<List<int>> store2Func;
      errModel.definedFunctions.TryGetValue("store2", out store2Func);
      if (store2Func != null) {
        foreach (List<int> store2FuncDef in store2Func) {
          // do not look at the else case of the function def, so check .Count
          if (store2FuncDef != null && store2FuncDef.Count == 5 && store2FuncDef[4] == id) {
            // make sure each predecessor is put into the list at most once
            if (!store2PredIdList.Contains(store2FuncDef[0])) {
              store2PredIdList.Add(store2FuncDef[0]);
            }
          }
        }
      }
      if (heapSuccPredIdList.Count + store2PredIdList.Count > 0) {
        if (store2PredIdList.Count == 1) {
          return store2PredIdList[0];
        } else if (store2PredIdList.Count == 0) {
          if (heapSuccPredIdList.Count == 1) {
            return heapSuccPredIdList[0];
          } else { //(heapSuccPredIdList.Count > 1)
            if (heapSuccPredIdList.Count == 2) {
              // if one of the 2 is a self-loop, take the other!
              if (heapSuccPredIdList[0] == id) {
                return heapSuccPredIdList[1];
              } else if (heapSuccPredIdList[1] == id) {
                return heapSuccPredIdList[0];
              } else {
                //no self-loop, two different predecessors, cannot choose
                return -1;
              }
            } else {
              // at least 3 different predecessors available, one of them could be a self-loop, but 
              // we cannot choose between the other 2 (or more) candidates
              return -1;
            }
          }
        } else {
          // more than one result in the succession coming from store2, no way 
          // to decide which is right, end here
          return -1;
        }
      } else {
        // no predecessor found
        return -1;
      }
    }

    static void TreatHeapSuccessions(List<int> heapSuccessionList, System.Collections.Hashtable incarnationMap, ErrorModel errModel,
      Dictionary<Incarnation, Absy/*!*/>/*!*/ incarnationOriginMap, List<string/*!*/>/*!*/ relatedInformation) {
      Contract.Requires(incarnationMap != null);
      Contract.Requires(errModel != null);
      Contract.Requires(cce.NonNullElements(incarnationOriginMap));
      Contract.Requires(cce.NonNullElements(relatedInformation));
      if (heapSuccessionList == null) {
        // empty list of heap successions, nothing we can do!
        return;
      }
      // primarily look for $o and $f (with skolem-id stuff) and then look where they were last changed!
      // find the o and f variables
      Variable dollarO = null;
      Variable dollarF = null;
      ICollection ic = incarnationMap.Keys;
      foreach (object o in ic) {
        if (o is BoundVariable) {
          BoundVariable bv = (BoundVariable)o;
          if (bv.Name == "$o") {
            dollarO = bv;
          } else if (bv.Name == "$f") {
            dollarF = bv;
          }
        }
      }
      if (dollarO == null || dollarF == null) {
        // without knowing the name of the current incarnation of $Heap, $o and $f we don't do anything here
      } else {
        object objO = incarnationMap[dollarO];
        object objF = incarnationMap[dollarF];
        if (objO != null && objF != null) {
          int zidO = errModel.identifierToPartition[objO.ToString()];
          int zidF = errModel.identifierToPartition[objF.ToString()];

          List<List<int>> select2Func = null;
          if (errModel.definedFunctions.TryGetValue("select2", out select2Func) && select2Func != null) {
            // check for all changes to $o.$f!
            List<int> heapsChangedOFZid = new List<int>();
            int oldValueZid = -1;
            int newValueZid = -1;

            for (int i = 0; i < heapSuccessionList.Count; i++) {
              bool foundValue = false;
              foreach (List<int> f in select2Func) {
                if (f != null && f.Count == 4 && f[0] == heapSuccessionList[i] && f[1] == zidO && f[2] == zidF) {
                  newValueZid = f[3];
                  foundValue = true;
                  break;
                }
              }
              if (!foundValue) {
                //    get default of the function once Leo&Nikolaj have changed it so the default is type correct
                //    for now treat it as a -1 !
                // the last element of select2Func is the else case, it has only 1 element, so grab that:
                // newValueZid = (select2Func[select2Func.Count-1])[0];
                newValueZid = -1;
              }

              if (oldValueZid != newValueZid) {
                // there was a change here, record that in the list:
                if (oldValueZid != -1) {
                  // don't record a change at the "initial" location, which refers to the $Heap in
                  // its current incarnation, and is marked by the oldValueZid being uninitialized
                  heapsChangedOFZid.Add(heapSuccessionList[i - 1]);
                }
                oldValueZid = newValueZid;
              }
            }

            foreach (int id in heapsChangedOFZid) {
              //get the heap name out of the errModel for this zid:
              List<string> l = errModel.partitionToIdentifiers[id];
              Contract.Assert(l != null);
              List<string> heaps = new List<string>();
              if (l != null) {
                foreach (string s in l) {
                  if (s.StartsWith("$Heap")) {
                    heaps.Add(s);
                  }
                }
              }
              // easy case first:
              if (heaps.Count == 1) {
                string heapName = heaps[0];
                // we have a string with the name of the heap, but we need to get the 
                // source location out of a map that uses Incarnations!

                ICollection incOrgMKeys = incarnationOriginMap.Keys;
                foreach (Incarnation inc in incOrgMKeys) {
                  if (inc != null) {
                    if (inc.Name == heapName) {
                      Absy source = null;
                      incarnationOriginMap.TryGetValue(inc, out source);
                      if (source != null) {
                        if (source is Block) {
                          Block b = (Block)source;
                          string fileName = b.tok.filename;
                          if (fileName != null) {
                            fileName = fileName.Substring(fileName.LastIndexOf('\\') + 1);
                          }
                          relatedInformation.Add("(related information): Changed $o.$f here: " + fileName + "(" + b.tok.line + "," + b.tok.col + ")");
                        } else if (source is Cmd) {
                          Cmd c = (Cmd)source;
                          string fileName = c.tok.filename;
                          if (fileName != null) {
                            fileName = fileName.Substring(fileName.LastIndexOf('\\') + 1);
                          }
                          relatedInformation.Add("(related information) Changed $o.$f here: " + fileName + "(" + c.tok.line + "," + c.tok.col + ")");
                        } else {
                          Contract.Assert(false);
                          throw new cce.UnreachableException();
                        }
                      }
                    }
                  }
                }
              } else {
                // more involved! best use some of the above code and try to synchronize joint parts
                // here there is more than one "$Heap@i" in the partition, check if they all agree on one
                // source location or maybe if some of them are joins (i.e. blocks) that should be ignored
              }

            }
          }
        }
      }
    }

    static string FormatReasonString(string reason, List<string> holeFillers) {
      if (holeFillers != null) {
        // in case all elements of holeFillers are "<unknown>" we can not say anything useful
        // so just say nothing and return null
        bool allUnknown = true;
        foreach (string s in holeFillers) {
          if (s != "<unknown>") {
            allUnknown = false;
            break;
          }
        }
        if (allUnknown) {
          return null;
        }
        string[] a = holeFillers.ToArray();
        reason = string.Format(reason, a);
      }
      return reason;
    }

    static object ValueFromZID(ErrorModel errModel, int id) {
      Contract.Requires(errModel != null);
      return ValueFromZID(errModel, id, null);
    }

    static object ValueFromZID(ErrorModel errModel, int id, string searchForAlternate) {
      Contract.Requires(errModel != null);
      object o = errModel.partitionToValue[id];
      if (o != null) {
        return o;
      } else {
        // more elaborate scheme to find something better, as in: look at the identifiers that
        // this partition maps to, or similar things!

        //treatment for 'null':
        int idForNull = -1;
        if (errModel.valueToPartition.TryGetValue("nullObject", out idForNull) && idForNull == id) {
          return "nullObject";
        }

        string returnStr = null;

        // "good identifiers" if there is no value found are 'unique consts' or 
        // '$in' parameters; '$in' parameters are treated, unclear how to get 'unique const' info
        List<string> identifiers = errModel.partitionToIdentifiers[id];
        if (identifiers != null) {
          foreach (string s in identifiers) {
            Contract.Assert(s != null);
            //$in parameters are (more) interesting than other identifiers, return the
            // first one found
            if (s.EndsWith("$in")) {
              returnStr = s;
              break;
            }
          }
        }

        // try to get mappings from one identifier to another if there are exactly 
        // two identifiers in the partition, where one of them is the identifier for which 
        // we search for alternate encodings (third parameter of the method) [or an incarnation
        // of such an identifier]
        if (returnStr == null && searchForAlternate != null && identifiers != null && identifiers.Count == 2) {
          if (identifiers[0] == searchForAlternate || identifiers[0].StartsWith(searchForAlternate + ".sk.")) {
            returnStr = identifiers[1];
          } else if (identifiers[1] == searchForAlternate || identifiers[1].StartsWith(searchForAlternate + ".sk.")) {
            returnStr = identifiers[0];
          }
        }

        if (returnStr != null) {
          return Helpers.BeautifyBplString(returnStr);
        }

        return null;
      }
    }

    static int TreatInterpretedFunction(string functionName, List<int> zargs, ErrorModel errModel) {
      Contract.Requires(functionName != null);
      Contract.Requires(zargs != null);
      Contract.Requires(errModel != null);
      if (zargs.Count != 2) {
        //all interpreted functions are binary, so there have to be exactly two arguments
        return -1;
      } else {
        object arg0 = ValueFromZID(errModel, zargs[0]);
        object arg1 = ValueFromZID(errModel, zargs[1]);
        if (arg0 is BigNum && arg1 is BigNum) {
          BigNum arg0i = (BigNum)arg0;
          BigNum arg1i = (BigNum)arg1;
          BigNum result;
          if (functionName == "+") {
            result = arg0i + arg1i;
          } else if (functionName == "-") {
            result = arg0i - arg1i;
          } else /*if (functionName == "*")*/ {
            result = arg0i * arg1i;
          }
          int returnId = -1;
          if (errModel.valueToPartition.TryGetValue(result, out returnId)) {
            return returnId;
          } else {
            return -1;
          }
        } else {
          //both arguments need to be integers for this to work!
          return -1;
        }
      }
    }

    static int TreatFunction(string functionName, List<int> zargs, bool undefined, ErrorModel errModel) {
      Contract.Requires(functionName != null);
      Contract.Requires(zargs != null);
      Contract.Requires(errModel != null);
      List<List<int>> functionDef;
      if ((!errModel.definedFunctions.TryGetValue(functionName, out functionDef) && functionName != "+" && functionName != "-" && functionName != "*") || undefined) {
        // no fitting function found or one of the arguments is undefined
        return -1;
      } else {
        if (functionName == "+" || functionName == "-" || functionName == "*") {
          return TreatInterpretedFunction(functionName, zargs, errModel);
        }
        Contract.Assert(functionDef != null);
        foreach (List<int> pWiseF in functionDef) {
          Contract.Assert(pWiseF != null);
          // else case in the function definition:
          if (pWiseF.Count == 1) {
            return pWiseF[0];
          }
          // number of arguments is exactly the right number
          Contract.Assert(zargs.Count == pWiseF.Count - 1);
          if (Contract.ForAll(zargs, i => i == pWiseF[i])) {
            return pWiseF[pWiseF.Count - 1];
          }
        }
        // all functions should have an 'else ->' case defined, therefore this should be
        // unreachable code!
        Contract.Assert(false);
        throw new cce.UnreachableException();
      }
    }

    //returned int is zID
    static int GetValuesFromModel(Bpl.Expr expr, Hashtable /*Variable -> Expr*/ incarnationMap, ErrorModel errModel,
      Dictionary<Bpl.Expr/*!*/, object>/*!*/ exprToPrintableValue) 
      //modifies exprToPrintableValue.*;
    {
      Contract.Requires(expr != null);
      Contract.Requires(incarnationMap != null);
      Contract.Requires(errModel != null);
      Contract.Requires(exprToPrintableValue != null);
      // call GetValuesFromModel on all proper subexpressions, returning their value, 
      // so they only have to be computed once!
      if (expr is LiteralExpr) {
        // nothing needs to be added to the exprToPrintableValue map, because e.g. 0 -> 0 is not interesting
        object o = ((LiteralExpr) expr).Val;
        if (o == null) {
          o = "nullObject";
        } 
        int idForExprVal;
        if (errModel.valueToPartition.TryGetValue(o, out idForExprVal)) {
          return idForExprVal;
        } else {
          return -1;
        }
      }
      else if (expr is IdentifierExpr) {
        // if the expression expr is in the incarnation map, then use its incarnation, 
        // otherwise just use the actual expression 
        string s = ((IdentifierExpr) expr).Name;
        object o = null;
        Variable v = ((IdentifierExpr) expr).Decl;
        if (v != null && incarnationMap.ContainsKey(v)) {
          if (incarnationMap[v] is IdentifierExpr) {
            s = ((IdentifierExpr) incarnationMap[v]).Name;
          } else if (incarnationMap[v] is LiteralExpr) {
            o = ((LiteralExpr) incarnationMap[v]).Val;
          }
        }
        // if o is not null, then we got a LiteralExpression, that needs separate treatment
        if (o == null) {
          // if the expression (respectively its incarnation) is mapped to some partition
          // then return that id, else return the error code -1
          if (errModel.identifierToPartition.ContainsKey(s)) {
            int i = errModel.identifierToPartition[s];
            // if the key is already in the map we can assume that it is the same map we would
            // get now and thus just ignore it
            if (!exprToPrintableValue.ContainsKey(expr)) {
              exprToPrintableValue.Add(expr, ValueFromZID(errModel, i, ((IdentifierExpr) expr).Name));
            }
            return i;
          } else {
            return -1;
          }
        } else if (errModel.valueToPartition.ContainsKey(o)) {
          int i = errModel.valueToPartition[o];
          if (!exprToPrintableValue.ContainsKey(expr))
            exprToPrintableValue.Add(expr, ValueFromZID(errModel, i));
          return i;
        } else {
          return -1;
        }
      }
      else if (expr is Bpl.NAryExpr) {
        Bpl.NAryExpr e = (Bpl.NAryExpr)expr;
        List<int> zargs = new List<int>();
        bool undefined = false;
        // do the recursion first
        foreach (Expr argument in ((NAryExpr) expr).Args) {
          int zid = -1;
          if (argument != null) {
            zid = GetValuesFromModel(argument, incarnationMap, errModel, exprToPrintableValue);
          }
          // if one of the arguments is 'undefined' then return -1 ('noZid') for this
          // but make sure the recursion is complete first1
          if (zid == -1) {
            undefined = true;
          }
          zargs.Add(zid);
        }
        IAppliable fun = e.Fun;
        Contract.Assert(fun != null);
        string functionName = fun.FunctionName; // PR: convert to select1, select2, etc in case of a map?
        // same as IndexedExpr:
        int id = TreatFunction(functionName, zargs, undefined, errModel);
        if (id != -1 && !exprToPrintableValue.ContainsKey(expr)) {
          exprToPrintableValue.Add(expr, ValueFromZID(errModel, id));
        }
        return id;
      } 
      else if (expr is Bpl.OldExpr) {
        //Bpl.OldExpr e = (Bpl.OldExpr)expr;
        // We do not know which heap is the old heap and what the latest state change was,
        // therefore we cannot return anything useful here!
        return -1;
      } 
      else if (expr is Bpl.QuantifierExpr) {
        Bpl.QuantifierExpr q = (Bpl.QuantifierExpr)expr;
        for (int i = 0; i < q.Dummies.Length; i++) {
          Bpl.Variable v = q.Dummies[i];
          if (v != null) {
            // add to the incarnation map a connection between the bound variable v
            // of the quantifier and its skolemized incarnation, if available,
            // i.e., search through the list of identifiers in the model and look for
            // v.sk.(q.SkolemId), only pick those that are directly associated to a value
            // DISCLAIMER: of course it is very well possible that one of these incarnations 
            // could be used without actually having a value, but it seems better to pick those
            // with a value, that is they are more likely to contribute useful information to 
            // the output
            List<Bpl.IdentifierExpr> quantVarIncarnationList = new List<Bpl.IdentifierExpr>();
            List<int> incarnationZidList = new List<int>();
            int numberOfNonNullValueIncarnations = 0;
            for (int j = 0; j < errModel.partitionToIdentifiers.Count; j++){
              List<string> pti = errModel.partitionToIdentifiers[j];
              Contract.Assert(pti != null);
              if (pti != null) {
                for (int k = 0; k < pti.Count; k++) {
                  // look for v.sk.(q.SkolemId)
                  // if there is more than one look at if there is exactly one with a non-null value
                  // associated, see above explanation
                  if (pti[k].StartsWith(v + ".sk." + q.SkolemId) && 
                      errModel.partitionToValue[errModel.identifierToPartition[pti[k]]] != null) {
                    quantVarIncarnationList.Add(new Bpl.IdentifierExpr(Bpl.Token.NoToken, pti[k], new Bpl.UnresolvedTypeIdentifier(Token.NoToken, "TName")));
                    incarnationZidList.Add(j);
                    if (errModel.partitionToValue[errModel.identifierToPartition[pti[k]]] != null) {
                      numberOfNonNullValueIncarnations++;
                    }
                  }
                }
              }
            }
            // only one such variable found, associate it with v
            if (quantVarIncarnationList.Count == 1) {
              incarnationMap[v] = quantVarIncarnationList[0];
            } else if (quantVarIncarnationList.Count > 1 && numberOfNonNullValueIncarnations == 1) {
              // if there are multiple candidate incarnations and exactly one of them has a value 
              // we can pick that one; otherwise it is not clear how to pick one out of multiple 
              // incarnations without a value or out of multiple incarnations with a value associated
              for (int n = 0; n < incarnationZidList.Count; n++) {
                if (errModel.partitionToValue[incarnationZidList[n]] != null) {
                  // quantVarIncarnationList and incarnationZidList are indexed in lockstep, so if
                  // for the associated zid the partitionToValue map is non-null then that is the one
                  // thus distinguished incarnation we want to put into the incarnationMap
                  incarnationMap[v] = quantVarIncarnationList[n];
                  break;
                }
              }
            }
          }
        }
        // generate the value of the body but do not return that outside
        GetValuesFromModel(q.Body, incarnationMap, errModel, exprToPrintableValue);
        // the quantifier cannot contribute any one value to the rest of the 
        // expression, thus just return -1
        return -1;
      } 
      else if (expr is Bpl.BvExtractExpr) {
        Bpl.BvExtractExpr ex = (Bpl.BvExtractExpr) expr;
        Bpl.Expr e0 = ex.Bitvector;
        Bpl.Expr e1 = new LiteralExpr(Token.NoToken, BigNum.FromInt(ex.Start));
        Bpl.Expr e2 = new LiteralExpr(Token.NoToken, BigNum.FromInt(ex.End));
        string functionName = "$bv_extract";
        List<int> zargs = new List<int>();
        bool undefined = false;
        
        int zid = -1;
        zid = GetValuesFromModel(e0, incarnationMap, errModel, exprToPrintableValue);
        if (zid == -1) {
          undefined = true;
        }
        zargs.Add(zid);
        
        zid = -1;
        zid = GetValuesFromModel(e1, incarnationMap, errModel, exprToPrintableValue);
        if (zid == -1) {
          undefined = true;
        }
        zargs.Add(zid);
        
        zid = -1;
        zid = GetValuesFromModel(e2, incarnationMap, errModel, exprToPrintableValue);
        if (zid == -1) {
          undefined = true;
        }
        zargs.Add(zid);
        
        //same as NAryExpr:
        int id = TreatFunction(functionName, zargs, undefined, errModel);
        if (id != -1 && !exprToPrintableValue.ContainsKey(expr)) {
          exprToPrintableValue.Add(expr, ValueFromZID(errModel, id));
        }
        return id;
      } 
      else if (expr is Bpl.BvConcatExpr) {
        // see comment above
        Bpl.BvConcatExpr bvc = (Bpl.BvConcatExpr) expr;
        string functionName = "$bv_concat";
        List<int> zargs = new List<int>();
        bool undefined = false;
        
        int zid = -1;
        zid = GetValuesFromModel(bvc.E0, incarnationMap, errModel, exprToPrintableValue);
        if (zid == -1) {
          undefined = true;
        }
        zargs.Add(zid);
        
        zid = -1;
        zid = GetValuesFromModel(bvc.E0, incarnationMap, errModel, exprToPrintableValue);
        if (zid == -1) {
          undefined = true;
        }
        zargs.Add(zid);
      
        //same as NAryExpr:
        int id = TreatFunction(functionName, zargs, undefined, errModel);
        if (id != -1 && !exprToPrintableValue.ContainsKey(expr)) {
          exprToPrintableValue.Add(expr, ValueFromZID(errModel, id));
        }
        return id;
      }
      else {
        Contract.Assert(false);throw new cce.UnreachableException();  // unexpected Bpl.Expr
      }
    }

    protected static Counterexample AssertCmdToCounterexample(AssertCmd cmd, TransferCmd transferCmd, BlockSeq trace, ErrorModel errModel, ModelViewInfo mvInfo, ProverContext context,
      Dictionary<Incarnation, Absy> incarnationOriginMap) 
    {
      Contract.Requires(cmd != null);
      Contract.Requires(transferCmd != null);
      Contract.Requires(trace != null);
      Contract.Requires(context != null);
      Contract.Requires(cce.NonNullElements(incarnationOriginMap));
      Contract.Ensures(Contract.Result<Counterexample>() != null);

      List<string> relatedInformation = new List<string>();
      if (CommandLineOptions.Clo.EnhancedErrorMessages == 1) {
        if (cmd.OrigExpr != null && cmd.IncarnationMap != null && errModel != null) {
          
          // get all possible information first
          Dictionary<Expr, object> exprToPrintableValue = new Dictionary<Expr, object>();
          GetValuesFromModel(cmd.OrigExpr, cmd.IncarnationMap, errModel, exprToPrintableValue);
          // then apply the strategies
          ApplyEnhancedErrorPrintingStrategy(cmd.OrigExpr, cmd.IncarnationMap, cmd.ErrorDataEnhanced, errModel, exprToPrintableValue, relatedInformation, true, incarnationOriginMap);
        }
      }
      
      // See if it is a special assert inserted in translation
      if (cmd is AssertRequiresCmd)
      {
        AssertRequiresCmd assertCmd = (AssertRequiresCmd)cmd;
        Contract.Assert(assertCmd != null);
        CallCounterexample cc = new CallCounterexample(trace, assertCmd.Call, assertCmd.Requires, errModel, mvInfo, context);
        cc.relatedInformation = relatedInformation;
        return cc;
      }
      else if (cmd is AssertEnsuresCmd)
      {
        AssertEnsuresCmd assertCmd = (AssertEnsuresCmd)cmd;
        Contract.Assert(assertCmd != null);
        ReturnCounterexample rc = new ReturnCounterexample(trace, transferCmd, assertCmd.Ensures, errModel, mvInfo, context);
        rc.relatedInformation = relatedInformation;
        return rc;
      }
      else 
      {
        AssertCounterexample ac = new AssertCounterexample(trace, (AssertCmd)cmd, errModel, mvInfo, context);
        ac.relatedInformation = relatedInformation;
        return ac;
      }
    }

    //    static void EmitImpl(Implementation! impl, bool printDesugarings) {
    //      int oldPrintUnstructured = CommandLineOptions.Clo.PrintUnstructured;
    //      CommandLineOptions.Clo.PrintUnstructured = 2;  // print only the unstructured program
    //      bool oldPrintDesugaringSetting = CommandLineOptions.Clo.PrintDesugarings;
    //      CommandLineOptions.Clo.PrintDesugarings = printDesugarings;
    //      impl.Emit(new TokenTextWriter("<console>", Console.Out, false), 0);
    //      CommandLineOptions.Clo.PrintDesugarings = oldPrintDesugaringSetting;
    //      CommandLineOptions.Clo.PrintUnstructured = oldPrintUnstructured;
    //    }

    static VCExpr LetVC(Block startBlock,
                        Variable controlFlowVariable,
                        Hashtable/*<int, Absy!>*/ label2absy,
                        ProverContext proverCtxt) {
      Contract.Requires(startBlock != null);
      Contract.Requires(controlFlowVariable != null);
      Contract.Requires(label2absy != null);
      Contract.Requires(proverCtxt != null);

      Contract.Ensures(Contract.Result<VCExpr>() != null);

      Hashtable/*<Block, LetVariable!>*/ blockVariables = new Hashtable/*<Block, LetVariable!!>*/();
      List<VCExprLetBinding> bindings = new List<VCExprLetBinding>();
      VCExpr startCorrect = LetVC(startBlock, controlFlowVariable, label2absy, blockVariables, bindings, proverCtxt);
      return proverCtxt.ExprGen.Let(bindings, startCorrect);
    }

    static VCExpr LetVC(Block block,
                        Variable controlFlowVariable,
                        Hashtable/*<int, Absy!>*/ label2absy,
                        Hashtable/*<Block, VCExprVar!>*/ blockVariables,
                        List<VCExprLetBinding/*!*/>/*!*/ bindings,
                        ProverContext proverCtxt)
    {
      Contract.Requires(block != null);
      Contract.Requires(controlFlowVariable != null);
      Contract.Requires(label2absy != null);
      Contract.Requires(blockVariables!= null);
      Contract.Requires(cce.NonNullElements(bindings));
      Contract.Requires(proverCtxt != null);

      Contract.Ensures(Contract.Result<VCExpr>() != null);

      VCExpressionGenerator gen = proverCtxt.ExprGen;
      Contract.Assert(gen != null);
      VCExprVar v = (VCExprVar)blockVariables[block];
      if (v == null) {
        /*
         * For block A (= block), generate:
         *   LET_binding A_correct = wp(A_body, (/\ S \in Successors(A) :: S_correct))
         * with the side effect of adding the let bindings to "bindings" for any
         * successor not yet visited.
         */
        VCExpr SuccCorrect;
        GotoCmd gotocmd = block.TransferCmd as GotoCmd;
        if (gotocmd == null) {
          ReturnExprCmd re = block.TransferCmd as ReturnExprCmd;
          if (re == null) {
            SuccCorrect = VCExpressionGenerator.True;
          } else {
            SuccCorrect = proverCtxt.BoogieExprTranslator.Translate(re.Expr);
          }
        } else {
          Contract.Assert( gotocmd.labelTargets != null);
          List<VCExpr> SuccCorrectVars = new List<VCExpr>(gotocmd.labelTargets.Length);
          foreach (Block successor in gotocmd.labelTargets) {Contract.Assert(successor != null);
            VCExpr s = LetVC(successor, controlFlowVariable, label2absy, blockVariables, bindings, proverCtxt);
            if (controlFlowVariable != null) 
            {
              VCExprVar controlFlowVariableExpr = proverCtxt.BoogieExprTranslator.LookupVariable(controlFlowVariable);
              VCExpr controlFlowFunctionAppl = gen.ControlFlowFunctionApplication(controlFlowVariableExpr, gen.Integer(BigNum.FromInt(block.UniqueId)));
              VCExpr controlTransferExpr = gen.Eq(controlFlowFunctionAppl, gen.Integer(BigNum.FromInt(successor.UniqueId)));
              s = gen.Implies(controlTransferExpr, s);
            }  
            SuccCorrectVars.Add(s);
          }
          SuccCorrect = gen.NAry(VCExpressionGenerator.AndOp, SuccCorrectVars);
        }

        
        VCContext context = new VCContext(label2absy, proverCtxt, controlFlowVariable);
        VCExpr vc = Wlp.Block(block, SuccCorrect, context);
        
        v = gen.Variable(block.Label + "_correct", Bpl.Type.Bool);
        bindings.Add(gen.LetBinding(v, vc));
        blockVariables.Add(block, v);
      }
      return v;
    }

    static VCExpr DagVC(Block block,
                         Hashtable/*<int, Absy!>*/ label2absy,
                         Hashtable/*<Block, VCExpr!>*/ blockEquations,
                         ProverContext proverCtxt)
    {
      Contract.Requires(block != null);
      Contract.Requires(label2absy != null);
      Contract.Requires(blockEquations != null);
      Contract.Requires(proverCtxt != null);
      Contract.Ensures(Contract.Result<VCExpr>() != null);

      VCExpressionGenerator gen = proverCtxt.ExprGen;
      Contract.Assert(gen != null);
      VCExpr vc = (VCExpr)blockEquations[block];
      if (vc != null) {
        return vc;
      }

      /* 
       * For block A (= block), generate:
       *   wp(A_body, (/\ S \in Successors(A) :: DagVC(S)))
       */
      VCExpr SuccCorrect = null;
      GotoCmd gotocmd = block.TransferCmd as GotoCmd;
      if (gotocmd != null)
      {
        foreach (Block successor in cce.NonNull(gotocmd.labelTargets)) {
          Contract.Assert(successor != null);
          VCExpr c = DagVC(successor, label2absy, blockEquations, proverCtxt);
          SuccCorrect = SuccCorrect == null ? c : gen.And(SuccCorrect, c);
        }
      }
      if (SuccCorrect == null) {
        SuccCorrect = VCExpressionGenerator.True;
      }

      VCContext context = new VCContext(label2absy, proverCtxt);
      vc = Wlp.Block(block, SuccCorrect, context);
      
      //  gen.MarkAsSharedFormula(vc);  PR: don't know yet what to do with this guy

      blockEquations.Add(block, vc);
      return vc;
    }

    static VCExpr FlatBlockVC(Implementation impl,
                              Hashtable/*<int, Absy!>*/ label2absy,
                              bool local, bool reach, bool doomed,
                              ProverContext proverCtxt)
    {
      Contract.Requires(impl != null);
      Contract.Requires(label2absy != null);
      Contract.Requires(proverCtxt != null);
      Contract.Requires( !local || !reach);  // "reach" must be false for local
    
      VCExpressionGenerator gen = proverCtxt.ExprGen;
      Contract.Assert(gen != null);
      Hashtable/* Block --> VCExprVar */ BlkCorrect = BlockVariableMap(impl.Blocks, "_correct", gen);
      Hashtable/* Block --> VCExprVar */ BlkReached = reach ? BlockVariableMap(impl.Blocks, "_reached", gen) : null;

      List<Block> blocks = impl.Blocks;
      Contract.Assert(blocks != null);
  // block sorting is now done on the VCExpr
  //    if (!local && (cce.NonNull(CommandLineOptions.Clo.TheProverFactory).NeedsBlockSorting) {
  //      blocks = SortBlocks(blocks);
  //    }

      VCExpr proofObligation;
      if (!local) {
        proofObligation = cce.NonNull((VCExprVar)BlkCorrect[impl.Blocks[0]]);
      } else {
        List<VCExpr> conjuncts = new List<VCExpr>(blocks.Count);
        foreach (Block b in blocks) {Contract.Assert(b != null);
          VCExpr v = cce.NonNull((VCExprVar)BlkCorrect[b]);
          conjuncts.Add(v);
        }
        proofObligation = gen.NAry(VCExpressionGenerator.AndOp, conjuncts);
      }

      VCContext context = new VCContext(label2absy, proverCtxt);
      Contract.Assert(context != null);

      List<VCExprLetBinding> programSemantics = new List<VCExprLetBinding>(blocks.Count);
      foreach (Block b in blocks) {Contract.Assert(b != null);
        /* 
         * In block mode,
         * For a return block A, generate:
         *   A_correct <== wp(A_body, true)  [post-condition has been translated into an assert]
         * For all other blocks, generate:
         *   A_correct <== wp(A_body, (/\ S \in Successors(A) :: S_correct))
         * 
         * In doomed mode, proceed as in block mode, except for a return block A, generate:
         *   A_correct <== wp(A_body, false)  [post-condition has been translated into an assert]
         *
         * In block reach mode, the wp(A_body,...) in the equations above change to:
         *   A_reached ==> wp(A_body,...)
         * and the conjunction above changes to:
         *   (/\ S \in Successors(A) :: S_correct \/ (\/ T \in Successors(A) && T != S :: T_reached))
         *
         * In local mode, generate:
         *   A_correct <== wp(A_body, true)
         */
        VCExpr SuccCorrect;
        if (local) {
          SuccCorrect = VCExpressionGenerator.True;
        } else {
          SuccCorrect = SuccessorsCorrect(b, BlkCorrect, BlkReached, doomed, gen);
        }

        VCExpr wlp = Wlp.Block(b, SuccCorrect, context);
        if (BlkReached != null) {
          wlp = gen.Implies(cce.NonNull((VCExprVar)BlkReached[b]), wlp);
        }
        
        VCExprVar okVar = cce.NonNull((VCExprVar)BlkCorrect[b]);
        VCExprLetBinding binding = gen.LetBinding(okVar, wlp);
        programSemantics.Add(binding);
      }

      return gen.Let(programSemantics, proofObligation);
    }

    private static Hashtable/* Block --> VCExprVar */ BlockVariableMap(List<Block/*!*/>/*!*/ blocks, string suffix,
                                                                        Microsoft.Boogie.VCExpressionGenerator gen) {
      Contract.Requires(cce.NonNullElements(blocks));
      Contract.Requires(suffix != null);
      Contract.Requires(gen != null);
      Contract.Ensures(Contract.Result<Hashtable>() != null);

      Hashtable/* Block --> VCExprVar */ map = new Hashtable/* Block --> (Let)Variable */();
      foreach (Block b in blocks) {
        Contract.Assert(b != null);
        VCExprVar v = gen.Variable(b.Label + suffix, Bpl.Type.Bool);
        Contract.Assert(v != null);
        map.Add(b, v);
      }
      return map;
    }

    private static VCExpr SuccessorsCorrect(
        Block b,
        Hashtable/* Block --> VCExprVar */ BlkCorrect,
        Hashtable/* Block --> VCExprVar */ BlkReached,
        bool doomed,
        Microsoft.Boogie.VCExpressionGenerator gen) {
      Contract.Requires(b != null);
      Contract.Requires(BlkCorrect != null);
      Contract.Requires(gen != null);
      Contract.Ensures(Contract.Result<VCExpr>() != null);

      VCExpr SuccCorrect = null;
      GotoCmd gotocmd = b.TransferCmd as GotoCmd;
      if (gotocmd != null) {
        foreach (Block successor in cce.NonNull(gotocmd.labelTargets)) {
          Contract.Assert(successor != null);
          // c := S_correct
          VCExpr c = (VCExprVar)BlkCorrect[successor];
          Contract.Assert(c != null);
          if (BlkReached != null) {
            // c := S_correct \/ Sibling0_reached \/ Sibling1_reached \/ ...;
            foreach (Block successorSibling in gotocmd.labelTargets) {
              Contract.Assert(successorSibling != null);
              if (successorSibling != successor) {
                c = gen.Or(c, cce.NonNull((VCExprVar)BlkReached[successorSibling]));
              }
            }
          }
          SuccCorrect = SuccCorrect == null ? c : gen.And(SuccCorrect, c);
        }
      }
      if (SuccCorrect == null) {
        return VCExpressionGenerator.True;
      } else if (doomed) {
        return VCExpressionGenerator.False;
      } else {
        return SuccCorrect;
      }
    }

    static VCExpr NestedBlockVC(Implementation impl,
                                Hashtable/*<int, Absy!>*/ label2absy,
                                bool reach,
                                ProverContext proverCtxt){
      Contract.Requires(impl != null);
      Contract.Requires(label2absy != null);
      Contract.Requires(proverCtxt != null);
      Contract.Requires( impl.Blocks.Count != 0);
      Contract.Ensures(Contract.Result<VCExpr>() != null);

      VCExpressionGenerator gen = proverCtxt.ExprGen;
      Contract.Assert(gen != null);
      Graph<Block> g = Program.GraphFromImpl(impl);

      Hashtable/* Block --> VCExprVar */ BlkCorrect = BlockVariableMap(impl.Blocks, "_correct", gen);
      Hashtable/* Block --> VCExprVar */ BlkReached = reach ? BlockVariableMap(impl.Blocks, "_reached", gen) : null;

      Block startBlock = cce.NonNull( impl.Blocks[0]);
      VCExpr proofObligation = (VCExprVar)BlkCorrect[startBlock];
      Contract.Assert(proofObligation != null);
      VCContext context = new VCContext(label2absy, proverCtxt);
      
      Hashtable/*Block->int*/ totalOrder = new Hashtable/*Block->int*/();
      {
        List<Block> blocks = impl.Blocks;
        
     // block sorting is now done on the VCExpr
     //   if (((!)CommandLineOptions.Clo.TheProverFactory).NeedsBlockSorting) {
     //     blocks = SortBlocks(blocks);
     //   }
        int i = 0;
        foreach (Block b in blocks) {
          Contract.Assert(b != null);
          totalOrder[b] = i;
          i++;
        }
      }
      
      VCExprLetBinding programSemantics = NestedBlockEquation(cce.NonNull(impl.Blocks[0]), BlkCorrect, BlkReached, totalOrder, context, g, gen);
      List<VCExprLetBinding> ps = new List<VCExprLetBinding>(1);
      ps.Add(programSemantics);
      
      return gen.Let(ps, proofObligation);
    }

    private static VCExprLetBinding NestedBlockEquation(Block b,
        Hashtable/*Block-->VCExprVar*/ BlkCorrect,
        Hashtable/*Block-->VCExprVar*/ BlkReached,
        Hashtable/*Block->int*/ totalOrder,
        VCContext context,
        Graph<Block> g,
        Microsoft.Boogie.VCExpressionGenerator gen) {
      Contract.Requires(b != null);
      Contract.Requires(BlkCorrect != null);
      Contract.Requires(BlkReached != null);
      Contract.Requires(totalOrder != null);
      Contract.Requires(g != null);
      Contract.Requires(context != null);

      Contract.Ensures(Contract.Result<VCExprLetBinding>() != null);

      /*
      * For a block b, return:
      *   LET_BINDING b_correct = wp(b_body, X)
      * where X is:
      *   LET (THOSE d \in DirectDominates(b) :: BlockEquation(d))
      *   IN (/\ s \in Successors(b) :: s_correct)
      * 
      * When the VC-expression generator does not support LET expresions, this
      * will eventually turn into:
      *   b_correct <== wp(b_body, X)
      * where X is:
      *   (/\ s \in Successors(b) :: s_correct)
      *   <==
      *   (/\ d \in DirectDominatees(b) :: BlockEquation(d))
      *
      * In both cases above, if BlkReached is non-null, then the wp expression
      * is instead:
      *   b_reached ==> wp(b_body, X)
      */

      VCExpr SuccCorrect = SuccessorsCorrect(b, BlkCorrect, null, false, gen);
      Contract.Assert(SuccCorrect != null);

      List<VCExprLetBinding> bindings = new List<VCExprLetBinding>();
      foreach (Block dominee in GetSortedBlocksImmediatelyDominatedBy(g, b, totalOrder)) {
        Contract.Assert(dominee != null);
        VCExprLetBinding c = NestedBlockEquation(dominee, BlkCorrect, BlkReached, totalOrder, context, g, gen);
        bindings.Add(c);
      }

      VCExpr X = gen.Let(bindings, SuccCorrect);
      VCExpr wlp = Wlp.Block(b, X, context);
      if (BlkReached != null) {
        wlp = gen.Implies((VCExprVar)BlkReached[b], wlp);
        Contract.Assert(wlp != null);
      }
      VCExprVar okVar = cce.NonNull((VCExprVar)BlkCorrect[b]);
      return gen.LetBinding(okVar, wlp);
    }

    /// <summary>
    /// Returns a list of g.ImmediatelyDominatedBy(b), but in a sorted order, hoping to steer around
    /// the nondeterminism problems we've been seeing by using just this call.
    /// </summary>
    static List<Block/*!*/>/*!*/ GetSortedBlocksImmediatelyDominatedBy(Graph<Block>/*!*/ g, Block/*!*/ b, Hashtable/*Block->int*//*!*/ totalOrder) {
      Contract.Requires(g != null);
      Contract.Requires(b != null);
      Contract.Requires(totalOrder != null);
      Contract.Ensures(Contract.Result<List<Block>>() != null);

      List<Block> list = new List<Block>();
      foreach (Block dominee in g.ImmediatelyDominatedBy(b)) {
        Contract.Assert(dominee != null);
        list.Add(dominee);
      }
      list.Sort(new Comparison<Block>(delegate(Block x, Block y) {
        return (int)cce.NonNull(totalOrder[x]) - (int)cce.NonNull(totalOrder[y]);
      }));
      return list;
    }

    static VCExpr VCViaStructuredProgram
                  (Implementation impl, Hashtable/*<int, Absy!>*/ label2absy,
                   ProverContext proverCtxt)
    {
    Contract.Requires(impl != null);
    Contract.Requires(label2absy != null);
    Contract.Requires(proverCtxt != null);
    Contract.Ensures(Contract.Result<VCExpr>() != null);

      #region Convert block structure back to a "regular expression"
      RE r = DAG2RE.Transform(cce.NonNull(impl.Blocks[0]));
      Contract.Assert(r != null);
      #endregion

      VCContext ctxt = new VCContext(label2absy, proverCtxt);
      Contract.Assert(ctxt != null);
      #region Send wlp(program,true) to Simplify
      return Wlp.RegExpr(r, VCExpressionGenerator.True, ctxt);
      #endregion
    }

    /// <summary> 
    /// Remove the empty blocks reachable from the block.
    /// It changes the visiting state of the blocks, so that if you want to visit again the blocks, you have to reset them...
    /// </summary>
    static BlockSeq RemoveEmptyBlocks(Block b) {
      Contract.Requires(b != null);
      Contract.Ensures(Contract.Result<BlockSeq>() != null);

      Contract.Assert(b.TraversingStatus == Block.VisitState.ToVisit);
      Block renameInfo;
      BlockSeq retVal = removeEmptyBlocksWorker(b, true, out renameInfo);
      if (renameInfo != null && !b.tok.IsValid) {
        bool onlyAssumes = true;
        foreach (Cmd c in b.Cmds) {
          if (!(c is AssumeCmd)) {
            onlyAssumes = false;
            break;
          }
        }
        if (onlyAssumes) {
          b.tok = renameInfo.tok;
          b.Label = renameInfo.Label;
        }
      }
      return retVal;
    }

    /// <summary>
    /// For every not-yet-visited block n reachable from b, change n's successors to skip empty nodes.
    /// Return the *set* of blocks reachable from b without passing through a nonempty block.
    /// The target of any backedge is counted as a nonempty block.
    /// If renameInfoForStartBlock is non-null, it denotes an empty block with location information, and that
    /// information would be appropriate to display
    /// </summary>
    private static BlockSeq removeEmptyBlocksWorker(Block b, bool startNode, out Block renameInfoForStartBlock)
  {
      Contract.Requires(b != null);
      Contract.Ensures(Contract.ValueAtReturn(out renameInfoForStartBlock) == null || Contract.ValueAtReturn(out renameInfoForStartBlock).tok.IsValid);
      // ensures: b in result ==> renameInfoForStartBlock == null;
    
      renameInfoForStartBlock = null;
      BlockSeq bs = new BlockSeq();
      GotoCmd gtc = b.TransferCmd as GotoCmd;

      // b has no successors
      if (gtc == null || gtc.labelTargets == null || gtc.labelTargets.Length == 0) 
      {
        if (b.Cmds.Length != 0){ // only empty blocks are removed...
          bs.Add(b);
        } else if (b.tok.IsValid) {
          renameInfoForStartBlock = b;
        }
        return bs;
      }
      else if (b.TraversingStatus == Block.VisitState.ToVisit)  // if b has some successors and we have not seen it so far...
      { 
        b.TraversingStatus = Block.VisitState.BeingVisited;

        // Before recursing down to successors, make a sobering observation:
        // If b has no commands and is not the start node, then it will see
        // extinction (because it will not be included in the "return setOfSuccessors"
        // statement below).  In that case, if b has a location, then the location
        // information would be lost.  Hence, make an attempt to save the location
        // by pushing the location onto b's successor.  This can be done if (0) b has
        // exactly one successor, (1) that successor has no location of its own, and
        // (2) that successor has no other predecessors.
        if (b.Cmds.Length == 0 && !startNode) {
          // b is about to become extinct; try to save its name and location, if possible
          if (b.tok.IsValid && gtc.labelTargets.Length == 1) {
            Block succ = cce.NonNull(gtc.labelTargets[0]);
            if (!succ.tok.IsValid && succ.Predecessors.Length == 1) {
              succ.tok = b.tok;
              succ.Label = b.Label;
            }
          }
        }

        // recursively call this method on each successor
        // merge result into a *set* of blocks
        Dictionary<Block,bool> mergedSuccessors = new Dictionary<Block,bool>();
        int m = 0;  // in the following loop, set renameInfoForStartBlock to the value that all recursive calls agree on, if possible; otherwise, null
        foreach (Block dest in gtc.labelTargets){Contract.Assert(dest != null);
          Block renameInfo;
          BlockSeq ys = removeEmptyBlocksWorker(dest, false, out renameInfo);
          Contract.Assert(ys != null);
          if (m == 0) {
            renameInfoForStartBlock = renameInfo;
          } else if (renameInfoForStartBlock != renameInfo) {
            renameInfoForStartBlock = null;
          }
          foreach (Block successor in ys){
            if (!mergedSuccessors.ContainsKey(successor))
              mergedSuccessors.Add(successor,true);
          }
          m++;
        }
        b.TraversingStatus = Block.VisitState.AlreadyVisited;

        BlockSeq setOfSuccessors = new BlockSeq();
        foreach (Block d in mergedSuccessors.Keys)
          setOfSuccessors.Add(d);
        if (b.Cmds.Length == 0 && !startNode) {
          // b is about to become extinct
          if (b.tok.IsValid) {
            renameInfoForStartBlock = b;
          }
          return setOfSuccessors;
        }
        // otherwise, update the list of successors of b to be the blocks in setOfSuccessors
        gtc.labelTargets = setOfSuccessors;
        gtc.labelNames = new StringSeq();
        foreach (Block d in setOfSuccessors){
          Contract.Assert(d != null);
          gtc.labelNames.Add(d.Label);}
        if (!startNode) {
          renameInfoForStartBlock = null;
        }
        return new BlockSeq(b);
      }
      else // b has some successors, but we are already visiting it, or we have already visited it...
      {
        return new BlockSeq(b);
      }
    }

    static void DumpMap(Hashtable /*Variable->Expr*/ map) {
      Contract.Requires(map != null);
      foreach (DictionaryEntry de in map) {
        Variable v = (Variable)de.Key;
        Contract.Assert(v != null);
        Expr e = (Expr)de.Value;
        Contract.Assert(e != null);
        Console.Write("  ");
        v.Emit(new TokenTextWriter("<console>", Console.Out, false), 0);
        Console.Write("  --> ");
        e.Emit(new TokenTextWriter("<console>", Console.Out, false));
        Console.WriteLine();
      }
    }
  }
}