MultiSets.dfy(158,2): Error: a postcondition could not be proved on this return path
MultiSets.dfy(157,14): Related location: this is the postcondition that could not be proved
MultiSets.dfy(164,2): Error: a postcondition could not be proved on this return path
MultiSets.dfy(163,14): Related location: this is the postcondition that could not be proved
MultiSets.dfy(177,19): Error: new number of occurrences might be negative
MultiSets.dfy(268,23): Error: assertion might not hold
MultiSets.dfy(291,15): Error: assertion might not hold

Dafny program verifier finished with 34 verified, 5 errors
