Dafny: added "choose" operator on sets

2 files changed, 6 insertions, 10 deletions

diff --git a/Test/dafny1/Answer b/Test/dafny1/Answer
index d9b32305..f54c87fd 100644
--- a/Test/dafny1/Answer
+++ b/Test/dafny1/Answer
@@ -81,7 +81,7 @@ Dafny program verifier finished with 132 verified, 0 errors
 

 -------------------- Celebrity.dfy --------------------

 

-Dafny program verifier finished with 11 verified, 0 errors

+Dafny program verifier finished with 10 verified, 0 errors

 

 -------------------- UltraFilter.dfy --------------------

 

diff --git a/Test/dafny1/Celebrity.dfy b/Test/dafny1/Celebrity.dfy
index 77af9327..25e6cc5c 100644
--- a/Test/dafny1/Celebrity.dfy
+++ b/Test/dafny1/Celebrity.dfy
@@ -1,9 +1,5 @@
 // Celebrity example, inspired by the Rodin tutorial

 

-method Pick<T>(S: set<T>) returns (t: T);

-  requires S != {};

-  ensures t in S;

-

 static function method Knows<Person>(a: Person, b: Person): bool;

   requires a != b;  // forbid asking about the reflexive case

 

@@ -26,20 +22,20 @@ method FindCelebrity1<Person>(people: set<Person>, ghost c: Person) returns (r:
   ensures r == c;

 {

   var Q := people;

-  call x := Pick(Q);

+  var x := choose(Q);

   while (Q != {x})

     //invariant Q <= people;  // inv1 in Rodin's Celebrity_1, but it's not needed here

     invariant IsCelebrity(c, Q);  // inv2

     invariant x in Q;

     decreases Q;

   {

-    call y := Pick(Q - {x});

+    var y := choose(Q - {x});

     if (Knows(x, y)) {

       Q := Q - {x};  // remove_1

     } else {

       Q := Q - {y};  assert x in Q;  // remove_2

     }

-    call x := Pick(Q);

+    x := choose(Q);

   }

   r := x;

 }

@@ -48,7 +44,7 @@ method FindCelebrity2<Person>(people: set<Person>, ghost c: Person) returns (r:
   requires IsCelebrity(c, people);

   ensures r == c;

 {

-  call b := Pick(people);

+  var b := choose(people);

   var R := people - {b};

   while (R != {})

     invariant R <= people;  // inv1

@@ -58,7 +54,7 @@ method FindCelebrity2<Person>(people: set<Person>, ghost c: Person) returns (r:
 

     decreases R;

   {

-    call x := Pick(R);

+    var x := choose(R);

     if (Knows(x, b)) {

       R := R - {x};

     } else {