path: root/Source/VCExpr/VCExprASTVisitors.cs

//-----------------------------------------------------------------------------
//
// Copyright (C) Microsoft Corporation.  All Rights Reserved.
//
//-----------------------------------------------------------------------------
using System;
using System.Text;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using Microsoft.Contracts;
using Microsoft.Basetypes;

// Some visitor skeletons for the VCExpression AST

namespace Microsoft.Boogie.VCExprAST
{
  using Microsoft.Boogie;

  public interface IVCExprVisitor<Result, Arg> {
    Result Visit(VCExprLiteral! node, Arg arg);
    Result Visit(VCExprNAry! node, Arg arg);
    Result Visit(VCExprVar! node, Arg arg);
    Result Visit(VCExprQuantifier! node, Arg arg);
    Result Visit(VCExprLet! node, Arg arg);
  }

  public interface IVCExprOpVisitor<Result, Arg> {
    Result VisitNotOp      (VCExprNAry! node, Arg arg);
    Result VisitEqOp       (VCExprNAry! node, Arg arg);
    Result VisitNeqOp      (VCExprNAry! node, Arg arg);
    Result VisitAndOp      (VCExprNAry! node, Arg arg);
    Result VisitOrOp       (VCExprNAry! node, Arg arg);
    Result VisitImpliesOp  (VCExprNAry! node, Arg arg);
    Result VisitDistinctOp (VCExprNAry! node, Arg arg);
    Result VisitLabelOp    (VCExprNAry! node, Arg arg);
    Result VisitSelectOp   (VCExprNAry! node, Arg arg);
    Result VisitStoreOp    (VCExprNAry! node, Arg arg);
    Result VisitBvOp       (VCExprNAry! node, Arg arg);
    Result VisitBvExtractOp(VCExprNAry! node, Arg arg);
    Result VisitBvConcatOp (VCExprNAry! node, Arg arg);
    Result VisitAddOp            (VCExprNAry! node, Arg arg);
    Result VisitSubOp            (VCExprNAry! node, Arg arg);
    Result VisitMulOp            (VCExprNAry! node, Arg arg);
    Result VisitDivOp            (VCExprNAry! node, Arg arg);
    Result VisitModOp            (VCExprNAry! node, Arg arg);
    Result VisitLtOp             (VCExprNAry! node, Arg arg);
    Result VisitLeOp             (VCExprNAry! node, Arg arg);
    Result VisitGtOp             (VCExprNAry! node, Arg arg);
    Result VisitGeOp             (VCExprNAry! node, Arg arg);
    Result VisitSubtypeOp        (VCExprNAry! node, Arg arg);
    Result VisitSubtype3Op       (VCExprNAry! node, Arg arg);
    Result VisitBoogieFunctionOp (VCExprNAry! node, Arg arg);
    Result VisitIfThenElseOp     (VCExprNAry! node, Arg arg);
    Result VisitCustomOp         (VCExprNAry! node, Arg arg);
  }

  //////////////////////////////////////////////////////////////////////////////
  // Standard implementations that make it easier to create own visitors

  // Simple traversal of VCExprs. The Visit implementations work
  // recursively, apart from the implementation for VCExprNAry that
  // uses a stack when applied to nested nodes with the same
  // operator, e.g., (AND (AND (AND ...) ...) ...). This is necessary
  // to avoid stack overflows

  public abstract class TraversingVCExprVisitor<Result, Arg>
                        : IVCExprVisitor<Result, Arg> {
    protected abstract Result StandardResult(VCExpr! node, Arg arg);

    public Result Traverse(VCExpr! node, Arg arg) {
      return node.Accept(this, arg);
    }

    public virtual Result Visit(VCExprLiteral! node, Arg arg) {
      return StandardResult(node, arg);
    }

    public virtual Result Visit(VCExprNAry! node, Arg arg) {
      Result res = StandardResult(node, arg);

      if (node.TypeParamArity == 0) {
        VCExprOp! op = node.Op;
        
        IEnumerator enumerator = new VCExprNAryUniformOpEnumerator(node);
        enumerator.MoveNext();  // skip the node itself

        while (enumerator.MoveNext()) {
          VCExpr! expr = (VCExpr!)enumerator.Current;
          VCExprNAry naryExpr = expr as VCExprNAry;
          if (naryExpr == null || !naryExpr.Op.Equals(op)) {
            expr.Accept(this, arg);
          } else {
            StandardResult(expr, arg);
          }
        }
      } else {
        foreach (VCExpr! e in node)
          e.Accept(this, arg);
      }

      return res;
    }

    public virtual Result Visit(VCExprVar! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result Visit(VCExprQuantifier! node, Arg arg) {
      Result res = StandardResult(node, arg);
      foreach (VCTrigger! trigger in node.Triggers)
        foreach (VCExpr! expr in trigger.Exprs)
          expr.Accept(this, arg);
      node.Body.Accept(this, arg);
      return res;
    }
    public virtual Result Visit(VCExprLet! node, Arg arg) {
      Result res = StandardResult(node, arg);
      // visit the bound expressions first
      foreach (VCExprLetBinding! binding in node)
        binding.E.Accept(this, arg);
      node.Body.Accept(this, arg);
      return res;
    }
  }

  //////////////////////////////////////////////////////////////////////////////
  // Class to iterate over the nodes of a tree of VCExprNAry. This is
  // used to avoid handling such VCExpr recursively, which can easily
  // lead to stack overflows

  public class VCExprNAryEnumerator : IEnumerator {

    private readonly VCExprNAry! CompleteExpr;
    private VCExpr CurrentExpr = null;
    private readonly Stack<VCExpr!>! ExprTodo = new Stack<VCExpr!> ();
    
    public VCExprNAryEnumerator(VCExprNAry! completeExpr) {
      this.CompleteExpr = completeExpr;
      Stack<VCExpr!>! exprTodo = new Stack<VCExpr!> ();
      exprTodo.Push(completeExpr);
      ExprTodo = exprTodo;
    }

    // Method using which a subclass can decide whether the
    // subexpressions of an expression should be enumerated as well
    // The default is to enumerate all nodes
    protected virtual bool Descend(VCExprNAry! expr) {
      return true;
    }
    
    ////////////////////////////////////////////////////////////////////////////

    public bool MoveNext() {
      if (ExprTodo.Count == 0)
        return false;

      CurrentExpr = ExprTodo.Pop();
      VCExprNAry currentNAry = CurrentExpr as VCExprNAry;
      if (currentNAry != null && Descend(currentNAry)) {
        for (int i = currentNAry.Arity - 1; i >= 0; --i)
          ExprTodo.Push(currentNAry[i]);
      }
      
      return true;
    }

    public object Current {
      get {
        return (!)CurrentExpr;
    } }

    public void Reset() {
      ExprTodo.Clear();
      CurrentExpr = null;
      ExprTodo.Push(CompleteExpr);
    }
  }

  
  //////////////////////////////////////////////////////////////////////////////

  public class VCExprNAryUniformOpEnumerator : VCExprNAryEnumerator {
    private readonly VCExprOp! Op;
    public VCExprNAryUniformOpEnumerator(VCExprNAry! completeExpr) {
      base(completeExpr);
      this.Op = completeExpr.Op;
    }
    protected override bool Descend(VCExprNAry! expr) {
      return expr.Op.Equals(Op) &&
             // we never skip nodes with type parameters
             // (those are too interesting ...)
             expr.TypeParamArity == 0;
    }
  }

  //////////////////////////////////////////////////////////////////////////////
  // Visitor that knows about the variables bound at each location in a VCExpr

  public abstract class BoundVarTraversingVCExprVisitor<Result, Arg>
                        : TraversingVCExprVisitor<Result, Arg> {
    // Maps with all variables bound above a certain location in the VCExpression.
    // The value of the map tells how often a particular symbol was bound
    private readonly IDictionary<VCExprVar!, int>! BoundTermVarsDict =
      new Dictionary<VCExprVar!, int> ();
    private readonly IDictionary<TypeVariable!, int>! BoundTypeVarsDict =
      new Dictionary<TypeVariable!, int> ();

    protected ICollection<VCExprVar!>! BoundTermVars { get {
      return BoundTermVarsDict.Keys;
    } }
    protected ICollection<TypeVariable!>! BoundTypeVars { get {
      return BoundTypeVarsDict.Keys;
    } }

    private void AddBoundVar<T>(IDictionary<T!, int>! dict, T! sym) {
      int n;
      if (dict.TryGetValue(sym, out n))
        dict[sym] = n + 1;
      else
        dict[sym] = 1;
    }

    private void RemoveBoundVar<T>(IDictionary<T!, int>! dict, T! sym) {
      int n;
      bool b = dict.TryGetValue(sym, out n);
      assert b && n > 0;
      if (n == 1)
        dict.Remove(sym);
      else
        dict[sym] = n - 1;
    }

    public override Result Visit(VCExprQuantifier! node, Arg arg) {
      // we temporarily add bound (term and type) variables to the
      // corresponding lists
      foreach (VCExprVar! v in node.BoundVars)
        AddBoundVar<VCExprVar>(BoundTermVarsDict, v);
      foreach (TypeVariable! v in node.TypeParameters)
        AddBoundVar<TypeVariable>(BoundTypeVarsDict, v);

      Result res;
      try {
        res = VisitAfterBinding(node, arg);
      } finally {
        foreach (VCExprVar! v in node.BoundVars)
          RemoveBoundVar<VCExprVar>(BoundTermVarsDict, v);
        foreach (TypeVariable! v in node.TypeParameters)
          RemoveBoundVar<TypeVariable>(BoundTypeVarsDict, v);
      }
      return res;
    }
    public override Result Visit(VCExprLet! node, Arg arg) {
      // we temporarily add bound term variables to the
      // corresponding lists
      foreach (VCExprVar! v in node.BoundVars)
        AddBoundVar<VCExprVar>(BoundTermVarsDict, v);

      Result res;
      try {
        res = VisitAfterBinding(node, arg);
      } finally {
        foreach (VCExprVar! v in node.BoundVars)
          RemoveBoundVar<VCExprVar>(BoundTermVarsDict, v);
      }
      return res;
    }

    ////////////////////////////////////////////////////////////////////////////
    // The possibility is provided to look at a (quantifier or let) node
    // after its bound variables have been registered
    // (when overriding the normal visit-methods, the node will be visited
    // before the binding happens)

    protected virtual Result VisitAfterBinding(VCExprQuantifier! node, Arg arg) {
      return base.Visit(node, arg);
    }

    protected virtual Result VisitAfterBinding(VCExprLet! node, Arg arg) {
      return base.Visit(node, arg);
    }
  }

  ////////////////////////////////////////////////////////////////////////////
  // General visitor for recursively collecting information in a VCExpr.
  // As the visitor is not used anywhere for the time being, it maybe should
  // be removed

  public abstract class CollectingVCExprVisitor<Result, Arg>
                        : IVCExprVisitor<Result, Arg> {
    protected abstract Result CombineResults(List<Result>! results, Arg arg);

    public Result Collect(VCExpr! node, Arg arg) {
      return node.Accept(this, arg);
    }

    public virtual Result Visit(VCExprLiteral! node, Arg arg) {
      return CombineResults(new List<Result> (), arg);
    }
    public virtual Result Visit(VCExprNAry! node, Arg arg) {
      List<Result>! results = new List<Result> ();
      foreach (VCExpr! subnode in node)
        results.Add(subnode.Accept(this, arg));
      return CombineResults(results, arg);
    }
    public virtual Result Visit(VCExprVar! node, Arg arg) {
      return CombineResults(new List<Result> (), arg);
    }
    public virtual Result Visit(VCExprQuantifier! node, Arg arg) {
      List<Result>! result = new List<Result> ();
      result.Add(node.Body.Accept(this, arg));
      foreach (VCTrigger! trigger in node.Triggers)
        foreach (VCExpr! expr in trigger.Exprs)
          result.Add(expr.Accept(this, arg));
      return CombineResults(result, arg);
    }
    public virtual Result Visit(VCExprLet! node, Arg arg) {
      List<Result>! results = new List<Result> ();
      // visit the bound expressions first
      foreach (VCExprLetBinding! binding in node)
        results.Add(binding.E.Accept(this, arg));
      results.Add(node.Body.Accept(this, arg));
      return CombineResults(results, arg);
    }
  }

  ////////////////////////////////////////////////////////////////////////////

  public class SizeComputingVisitor : TraversingVCExprVisitor<bool, bool> {

    private int Size = 0;

    public static int ComputeSize(VCExpr! expr) {
      SizeComputingVisitor! visitor = new SizeComputingVisitor();
      visitor.Traverse(expr, true);
      return visitor.Size;
    }
    
    protected override bool StandardResult(VCExpr! node, bool arg) {
      Size = Size + 1;
      return true;
    }
  }

  ////////////////////////////////////////////////////////////////////////////

  // Collect all free term and type variables in a VCExpr. Type variables
  // can occur free either in the types of bound variables, or in the type
  // parameters of VCExprNAry.

  // the result and argument (of type bool) are not used currently
  public class FreeVariableCollector : BoundVarTraversingVCExprVisitor<bool, bool> {
    public readonly Dictionary<VCExprVar!,object>! FreeTermVars = new Dictionary<VCExprVar!,object> ();
    public readonly List<TypeVariable!>! FreeTypeVars = new List<TypeVariable!> ();

    // not used
    protected override bool StandardResult(VCExpr! node, bool arg) {
      return true;
    }

    public static Dictionary<VCExprVar!,object>! FreeTermVariables(VCExpr! node) {
      FreeVariableCollector collector = new FreeVariableCollector ();
      collector.Traverse(node, true);
      return collector.FreeTermVars;
    }

    public static List<TypeVariable!>! FreeTypeVariables(VCExpr! node) {
      FreeVariableCollector collector = new FreeVariableCollector ();
      collector.Traverse(node, true);
      return collector.FreeTypeVars;
    }

    public void Reset() {
      FreeTermVars.Clear();
      FreeTypeVars.Clear();
    }

    public void Collect(VCExpr! node) {
      Traverse(node, true);
    }

    public void Collect(Type! type) {
      AddTypeVariables(type.FreeVariables.ToList());
    }

    /////////////////////////////////////////////////////////////////////////

    private void CollectTypeVariables(IEnumerable<VCExprVar!>! boundVars) {
      foreach (VCExprVar! var in boundVars)
        Collect(var.Type);
    }

    private void AddTypeVariables(IEnumerable<TypeVariable!>! typeVars) {
      foreach (TypeVariable! tvar in typeVars)
        if (!BoundTypeVars.Contains(tvar) && !FreeTypeVars.Contains(tvar))
          FreeTypeVars.Add(tvar);
    }

    public override bool Visit(VCExprVar! node, bool arg) {
      if (!BoundTermVars.Contains(node) && !FreeTermVars.ContainsKey(node)) {
        FreeTermVars.Add(node, null);
        Collect(node.Type);
      }
      return true;
    }

    public override bool Visit(VCExprNAry! node, bool arg) {
      foreach (Type! t in node.TypeArguments)
        Collect(t);
      return base.Visit(node, arg);
    }

    protected override bool VisitAfterBinding(VCExprQuantifier! node, bool arg) {
      CollectTypeVariables(node.BoundVars);
      return base.VisitAfterBinding(node, arg);
    }

    protected override bool VisitAfterBinding(VCExprLet! node, bool arg) {
      CollectTypeVariables(node.BoundVars);
      return base.VisitAfterBinding(node, arg);
    }
  }

  ////////////////////////////////////////////////////////////////////////////
  // Framework for mutating VCExprs

  // The Visit implementations in the following visitor work
  // recursively, apart from the implementation for VCExprNAry that
  // uses its own stack when applied to nested nodes with the same
  // operator, e.g., (AND (AND (AND ...) ...) ...). This is necessary
  // to avoid stack overflows (like in TraversingVCExprVisitor)

  public abstract class MutatingVCExprVisitor<Arg>
                        : IVCExprVisitor<VCExpr!, Arg> {
    protected readonly VCExpressionGenerator! Gen;

    public MutatingVCExprVisitor(VCExpressionGenerator! gen) {
      this.Gen = gen;
    }

    public VCExpr! Mutate(VCExpr! expr, Arg arg) {
      return expr.Accept(this, arg);
    }

    public List<VCExpr!>! MutateSeq(IEnumerable<VCExpr!>! exprs, Arg arg) {
      List<VCExpr!>! res = new List<VCExpr!> ();
      foreach (VCExpr! expr in exprs)
        res.Add(expr.Accept(this, arg));
      return res;
    }

    private List<VCExpr!>! MutateList(List<VCExpr!>! exprs, Arg arg) {
      bool changed = false;
      List<VCExpr!>! res = new List<VCExpr!> ();
      foreach (VCExpr! expr in exprs) {
        VCExpr! newExpr = expr.Accept(this, arg);
        if (!Object.ReferenceEquals(expr, newExpr))
          changed = true;
        res.Add(newExpr);
      }
      if (!changed)
        return exprs;
      return res;
    }

    public virtual VCExpr! Visit(VCExprLiteral! node, Arg arg) {
      return node;
    }

    ////////////////////////////////////////////////////////////////////////////

    // Special element used to mark the positions in the todo-stack where
    // results have to be popped from the result-stack.
    private static readonly VCExpr! CombineResultsMarker = new VCExprLiteral (Type.Bool);

    // The todo-stack contains records of the shape
    //
    //     arg0
    //     arg1
    //     arg2
    //     ...
    //     CombineResultsMarker
    //     f(arg0, arg1, arg2, ...)               (the original expression)

    private readonly Stack<VCExpr!>! NAryExprTodoStack = new Stack<VCExpr!> ();
    private readonly Stack<VCExpr!>! NAryExprResultStack = new Stack<VCExpr!> ();

    private void PushTodo(VCExprNAry! exprTodo) {
      NAryExprTodoStack.Push(exprTodo);
      NAryExprTodoStack.Push(CombineResultsMarker);
      for (int i = exprTodo.Arity - 1; i >= 0; --i)
        NAryExprTodoStack.Push(exprTodo[i]);
    }

    public virtual VCExpr! Visit(VCExprNAry! node, Arg arg) {
      VCExprOp! op = node.Op;
      int initialStackSize = NAryExprTodoStack.Count;
      int initialResultStackSize = NAryExprResultStack.Count;

      PushTodo(node);
      
      while (NAryExprTodoStack.Count > initialStackSize) {
        VCExpr! subExpr = NAryExprTodoStack.Pop();
        
        if (Object.ReferenceEquals(subExpr, CombineResultsMarker)) {
          //
          // assemble a result
          VCExprNAry! originalExpr = (VCExprNAry)NAryExprTodoStack.Pop();
          bool changed = false;
          List<VCExpr!>! newSubExprs = new List<VCExpr!> ();

          for (int i = op.Arity - 1; i >= 0; --i) {
            VCExpr! nextSubExpr = NAryExprResultStack.Pop();
            if (!Object.ReferenceEquals(nextSubExpr, originalExpr[i]))
              changed = true;
            newSubExprs.Insert(0, nextSubExpr);
          }
          
          NAryExprResultStack.Push(UpdateModifiedNode(originalExpr, newSubExprs, changed, arg));
          //
        } else {
          //
          VCExprNAry narySubExpr = subExpr as VCExprNAry;
          if (narySubExpr != null && narySubExpr.Op.Equals(op) &&
              // as in VCExprNAryUniformOpEnumerator, all expressions with
              // type parameters are allowed to be inspected more closely
              narySubExpr.TypeParamArity == 0) {
            PushTodo(narySubExpr);
          } else {
            NAryExprResultStack.Push(subExpr.Accept(this, arg));
          }
          //
        }
      }

      assert NAryExprTodoStack.Count == initialStackSize &&
             NAryExprResultStack.Count == initialResultStackSize + 1;
      return NAryExprResultStack.Pop();
    }

    protected virtual VCExpr! UpdateModifiedNode(VCExprNAry! originalNode,
                                                 List<VCExpr!>! newSubExprs,
                                                 // has any of the subexpressions changed?
                                                 bool changed,
                                                 Arg arg) {
      if (changed)
        return Gen.Function(originalNode.Op,
                            newSubExprs, originalNode.TypeArguments);
      else
        return originalNode;
    }

    ////////////////////////////////////////////////////////////////////////////

    public virtual VCExpr! Visit(VCExprVar! node, Arg arg) {
      return node;
    }

    protected List<VCTrigger!>! MutateTriggers(List<VCTrigger!>! triggers, Arg arg) {
      List<VCTrigger!>! newTriggers = new List<VCTrigger!> ();
      bool changed = false;
      foreach (VCTrigger! trigger in triggers) {
        List<VCExpr!>! exprs = trigger.Exprs;
        List<VCExpr!>! newExprs = MutateList(exprs, arg);
        if (Object.ReferenceEquals(exprs, newExprs)) {
          newTriggers.Add(trigger);
        } else {
          newTriggers.Add(Gen.Trigger(trigger.Pos, newExprs));
          changed = true;
        }
      }
      if (!changed)
        return triggers;
      return newTriggers;
    }

    public virtual VCExpr! Visit(VCExprQuantifier! node, Arg arg) {
      bool changed = false;

      VCExpr! body = node.Body;
      VCExpr! newbody = body.Accept(this, arg);
      if (!Object.ReferenceEquals(body, newbody))
        changed = true;

      // visit the trigger expressions as well
      List<VCTrigger!>! triggers = node.Triggers;
      List<VCTrigger!>! newTriggers = MutateTriggers(triggers, arg);
      if (!Object.ReferenceEquals(triggers, newTriggers))
        changed = true;

      if (!changed)
        return node;
      return Gen.Quantify(node.Quan, node.TypeParameters, node.BoundVars,
                          newTriggers, node.Infos, newbody);
    }

    public virtual VCExpr! Visit(VCExprLet! node, Arg arg) {
      bool changed = false;

      VCExpr! body = node.Body;
      VCExpr! newbody = body.Accept(this, arg);
      if (!Object.ReferenceEquals(body, newbody))
        changed = true;

      List<VCExprLetBinding!>! newbindings = new List<VCExprLetBinding!> ();
      for (int i = 0; i < node.Length; ++i) {
        VCExprLetBinding! binding = node[i];
        VCExpr! e = binding.E;
        VCExpr! newE = e.Accept(this, arg);
        if (Object.ReferenceEquals(e, newE)) {
          newbindings.Add(binding);
        } else {
          changed = true;
          newbindings.Add(Gen.LetBinding(binding.V, newE));
        }
      }

      if (!changed)
        return node;
      return Gen.Let(newbindings, newbody);
    }
  }

  ////////////////////////////////////////////////////////////////////////////
  // Substitutions and a visitor for applying substitutions. A substitution can
  // substitute both type variables and term variables

  public class VCExprSubstitution {
    private readonly List<IDictionary<VCExprVar!, VCExpr!>!>! TermSubsts;
    private readonly List<IDictionary<TypeVariable!, Type!>!>! TypeSubsts;

    public VCExprSubstitution(IDictionary<VCExprVar!, VCExpr!>! termSubst,
                              IDictionary<TypeVariable!, Type!>! typeSubst) {
      List<IDictionary<VCExprVar!, VCExpr!>!>! termSubsts =
        new List<IDictionary<VCExprVar!, VCExpr!>!> ();
      termSubsts.Add(termSubst);
      List<IDictionary<TypeVariable!, Type!>!>! typeSubsts =
        new List<IDictionary<TypeVariable!, Type!>!> ();
      typeSubsts.Add(typeSubst);
      this.TermSubsts = termSubsts;
      this.TypeSubsts = typeSubsts;
    }

    public VCExprSubstitution() {
      this(new Dictionary<VCExprVar!, VCExpr!> (), new Dictionary<TypeVariable!, Type!> ());
    }

    public void PushScope() {
      TermSubsts.Add(new Dictionary<VCExprVar!, VCExpr!> ());
      TypeSubsts.Add(new Dictionary<TypeVariable!, Type!> ());
    }

    public void PopScope() {
      TermSubsts.RemoveAt(TermSubsts.Count - 1);
      TypeSubsts.RemoveAt(TypeSubsts.Count - 1);
    }

    public VCExpr this[VCExprVar! var] {
      get {
        VCExpr res;
        for (int i = TermSubsts.Count - 1; i >= 0; --i) {
          if (TermSubsts[i].TryGetValue(var, out res))
            return res;
        }
        return null;
      }
      set {
        TermSubsts[TermSubsts.Count - 1][var] = (!)value;
      }
    }

    public Type this[TypeVariable! var] {
      get {
        Type res;
        for (int i = TypeSubsts.Count - 1; i >= 0; --i) {
          if (TypeSubsts[i].TryGetValue(var, out res))
            return res;
        }
        return null;
      }
      set {
        TypeSubsts[TypeSubsts.Count - 1][var] = (!)value;
      }
    }

    public bool ContainsKey(VCExprVar! var) {
      return this[var] != null;
    }

    public bool ContainsKey(TypeVariable! var) {
      return this[var] != null;
    }

    public bool TermSubstIsEmpty { get {
        return forall{IDictionary<VCExprVar!, VCExpr!>! dict in TermSubsts;
                      dict.Count == 0};
      } }

    public bool TypeSubstIsEmpty { get {
        return forall{IDictionary<TypeVariable!, Type!>! dict in TypeSubsts;
                      dict.Count == 0};
      } }

    public IDictionary<TypeVariable!, Type!>! ToTypeSubst { get {
        IDictionary<TypeVariable!, Type!>! res = new Dictionary<TypeVariable!, Type!> ();
        foreach (IDictionary<TypeVariable!, Type!>! dict in TypeSubsts)
          foreach (KeyValuePair<TypeVariable!, Type!> pair in dict)
            // later ones overwrite earlier ones
            res[pair.Key] = pair.Value;
        return res;
      } }

    // the variables that are not mapped to themselves
    public IEnumerable<VCExprVar!>! TermDomain { get {
      Dictionary<VCExprVar!, bool>! domain = new Dictionary<VCExprVar!, bool> ();
      foreach (IDictionary<VCExprVar!, VCExpr!>! dict in TermSubsts)
        foreach (VCExprVar! var in dict.Keys)
          if (!var.Equals(this[var]))
            domain.Add(var, true);
      return domain.Keys;
    } }

    // the variables that are not mapped to themselves
    public IEnumerable<TypeVariable!>! TypeDomain { get {
      Dictionary<TypeVariable!, bool>! domain = new Dictionary<TypeVariable!, bool> ();
      foreach (IDictionary<TypeVariable!, Type!>! dict in TypeSubsts)
        foreach (TypeVariable! var in dict.Keys)
          if (!var.Equals(this[var]))
            domain.Add(var, true);
      return domain.Keys;
    } }

    public FreeVariableCollector! Codomains { get {
        FreeVariableCollector! coll = new FreeVariableCollector ();
        foreach (VCExprVar! var in TermDomain)
          coll.Collect((!)this[var]);
        foreach (TypeVariable! var in TypeDomain)
          coll.Collect((!)this[var]);
        return coll;
      } }

    public VCExprSubstitution! Clone() {
      VCExprSubstitution! res = new VCExprSubstitution ();
      foreach (IDictionary<VCExprVar!, VCExpr!>! dict in TermSubsts)
        res.TermSubsts.Add(HelperFuns.Clone(dict));
      foreach (IDictionary<TypeVariable!, Type!>! dict in TypeSubsts)
        res.TypeSubsts.Add(HelperFuns.Clone(dict));
      return res;
    }
  }

  /////////////////////////////////////////////////////////////////////////////////

  public class SubstitutingVCExprVisitor
               : MutatingVCExprVisitor<VCExprSubstitution!> {
    public SubstitutingVCExprVisitor(VCExpressionGenerator! gen) {
      base(gen);
    }

    // when descending across a binder, we have to check that no collisions
    // or variable capture can occur. if this might happen, we replace the
    // term and type variables bound by the binder with fresh variables
    private bool CollisionPossible(IEnumerable<TypeVariable!>! typeParams,
                                   IEnumerable<VCExprVar!>! boundVars,
                                   VCExprSubstitution! substitution) {
      // variables can be shadowed by a binder
      if (exists{TypeVariable! var in typeParams; substitution.ContainsKey(var)} ||
          exists{VCExprVar! var in boundVars; substitution.ContainsKey(var)})
        return true;
      // compute the codomain of the substitution
      FreeVariableCollector! coll = substitution.Codomains;
      // variables could be captured when applying the substitution
      return exists{TypeVariable! var in typeParams; coll.FreeTypeVars.Contains(var)} ||
             exists{VCExprVar! var in boundVars; coll.FreeTermVars.ContainsKey(var)};
    }

    // can be overwritten if names of bound variables are to be changed
    protected virtual string! ChooseNewVariableName(string! oldName) {
      return oldName;
    }

    // handle type parameters in VCExprNAry
    protected override VCExpr! UpdateModifiedNode(VCExprNAry! originalNode,
                                                  List<VCExpr!>! newSubExprs,
                                                  bool changed,
                                                  VCExprSubstitution! substitution) {
      List<Type!>! typeParams = new List<Type!> ();
      foreach (Type! t in originalNode.TypeArguments) {
        Type! newType = t.Substitute(substitution.ToTypeSubst);
        if (!ReferenceEquals(t, newType))
          changed = true;
        typeParams.Add(newType);
      }
      if (changed)
        return Gen.Function(originalNode.Op, newSubExprs, typeParams);
      else
        return originalNode;
    }

    public override VCExpr! Visit(VCExprQuantifier! node,
                                  VCExprSubstitution! substitution) {
      // the default is to refresh bound variables only if necessary
      // because of collisions
      return Visit(node, substitution, false);
    }

    public VCExpr! Visit(VCExprQuantifier! node,
                         VCExprSubstitution! substitution,
                         bool refreshBoundVariables) {
      substitution.PushScope(); try {

      List<TypeVariable!>! typeParams = node.TypeParameters;
      bool refreshAllVariables = refreshBoundVariables ||
                                 CollisionPossible(node.TypeParameters, node.BoundVars, substitution);
      if (refreshAllVariables) {
        // we introduce fresh type variables to ensure that none gets captured
        typeParams = new List<TypeVariable!> ();
        foreach (TypeVariable! var in node.TypeParameters) {
          TypeVariable! freshVar =
            new TypeVariable (Token.NoToken, ChooseNewVariableName(var.Name));
          typeParams.Add(freshVar);
          substitution[var] = freshVar;
          // this might overwrite other elements of the substitution, deliberately
        }
      }

      List<VCExprVar!>! boundVars = node.BoundVars;
      if (refreshAllVariables || !substitution.TypeSubstIsEmpty) {
        // collisions are possible, or we also substitute type variables. in this case
        // the bound term variables have to be replaced with fresh variables with the
        // right types
        boundVars = new List<VCExprVar!> ();
        IDictionary<TypeVariable!, Type!>! typeSubst = substitution.ToTypeSubst;
        foreach (VCExprVar! var in node.BoundVars) {
          VCExprVar! freshVar =
            Gen.Variable(ChooseNewVariableName(var.Name),
                         var.Type.Substitute(typeSubst));
          boundVars.Add(freshVar);
          substitution[var] = freshVar;
          // this might overwrite other elements of the substitution, deliberately
        }
      }

      List<VCTrigger!>! newTriggers = new List<VCTrigger!> ();
      foreach (VCTrigger! trigger in node.Triggers)
        newTriggers.Add(Gen.Trigger(trigger.Pos, MutateSeq(trigger.Exprs, substitution)));

      VCExpr! newBody = Mutate(node.Body, substitution);

      return Gen.Quantify(node.Quan, typeParams, boundVars,
                          newTriggers, node.Infos, newBody);

      } finally {
        substitution.PopScope();
      }
    }

    public override VCExpr! Visit(VCExprVar! node,
                                  VCExprSubstitution! substitution) {
      VCExpr res = substitution[node];
      if (res != null)
        return res;
      return node;
    }

    public override VCExpr! Visit(VCExprLet! node,
                                  VCExprSubstitution! substitution) {
      // the default is to refresh bound variables only if necessary
      // because of collisions
      return Visit(node, substitution, false);
    }

    public VCExpr! Visit(VCExprLet! node,
                         VCExprSubstitution! substitution,
                         bool refreshBoundVariables) {
      // let-expressions do not have type parameters (fortunately ...)
      substitution.PushScope (); try {

      bool refreshAllVariables =
        refreshBoundVariables ||
        !substitution.TypeSubstIsEmpty ||
        CollisionPossible(new List<TypeVariable!> (), node.BoundVars, substitution);

      List<VCExprVar!>! newBoundVars = node.BoundVars;
      if (refreshAllVariables) {
        // collisions are possible, or we also substitute type variables. in this case
        // the bound term variables have to be replaced with fresh variables with the
        // right types
        newBoundVars = new List<VCExprVar!> ();
        IDictionary<TypeVariable!, Type!>! typeSubst = substitution.ToTypeSubst;
        foreach (VCExprVar! var in node.BoundVars) {
          VCExprVar! freshVar =
            Gen.Variable(ChooseNewVariableName(var.Name),
                         var.Type.Substitute(typeSubst));
          newBoundVars.Add(freshVar);
          substitution[var] = freshVar;
          // this might overwrite other elements of the substitution, deliberately
        }
      }

      List<VCExprLetBinding!>! newbindings = new List<VCExprLetBinding!> ();
      for (int i = 0; i < node.Length; ++i) {
        VCExprLetBinding! binding = node[i];
        newbindings.Add(Gen.LetBinding(newBoundVars[i], Mutate(binding.E, substitution)));
      }
      
      VCExpr! newBody = Mutate(node.Body, substitution);
      return Gen.Let(newbindings, newBody);

      } finally {
        substitution.PopScope();
      }
    }
  }

  ////////////////////////////////////////////////////////////////////////////

  public abstract class StandardVCExprOpVisitor<Result, Arg>
                        : IVCExprOpVisitor<Result, Arg> {
    protected abstract Result StandardResult(VCExprNAry! node, Arg arg);

    public virtual Result VisitNotOp      (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitEqOp       (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitNeqOp      (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitAndOp      (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitOrOp       (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitImpliesOp  (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitDistinctOp (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitLabelOp    (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitSelectOp   (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitStoreOp    (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitBvOp       (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitBvExtractOp(VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitBvConcatOp (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitIfThenElseOp (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitCustomOp     (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitAddOp            (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitSubOp            (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitMulOp            (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitDivOp            (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitModOp            (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitLtOp             (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitLeOp             (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitGtOp             (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitGeOp             (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitSubtypeOp        (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitSubtype3Op       (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
    public virtual Result VisitBoogieFunctionOp (VCExprNAry! node, Arg arg) {
      return StandardResult(node, arg);
    }
  }

}