MultiSets.dfy(159,2): Error BP5003: A postcondition might not hold on this return path.
MultiSets.dfy(158,14): Related location: This is the postcondition that might not hold.
Execution trace:
    (0,0): anon0
MultiSets.dfy(165,2): Error BP5003: A postcondition might not hold on this return path.
MultiSets.dfy(164,14): Related location: This is the postcondition that might not hold.
Execution trace:
    (0,0): anon0
MultiSets.dfy(178,10): Error: new number of occurrences might be negative
Execution trace:
    (0,0): anon0
    (0,0): anon4_Then
    (0,0): anon3
MultiSets.dfy(269,23): Error: assertion violation
Execution trace:
    (0,0): anon0
    (0,0): anon11_Then
    (0,0): anon3
    (0,0): anon12_Then
    (0,0): anon14_Else
MultiSets.dfy(292,15): Error: assertion violation
Execution trace:
    (0,0): anon0
    (0,0): anon8_Then
    (0,0): anon3
    (0,0): anon9_Else

Dafny program verifier finished with 59 verified, 5 errors