git-issue-4787.dfy(11,2): Error: a postcondition could not be proved on this return path
   |
11 |   {}
   |   ^

git-issue-4787.dfy(10,12): Related location: this is the postcondition that could not be proved
   |
10 |     ensures |ToSet(xs)| <= |xs|
   |             ^^^^^^^^^^^^^^^^^^^

git-issue-4787.dfy(14,13): Warning: unnecessary requires clause
   |
14 |     requires xs == []
   |              ^^^^^^^^

git-issue-4787.dfy(15,12): Warning: ensures clause proved using contradictory assumptions
   |
15 |     ensures |ToSet(xs)| == 0
   |             ^^^^^^^^^^^^^^^^


Dafny program verifier finished with 1 verified, 1 error
