Twice.dfy(27,22): Error: assertion violation
Execution trace:
    (0,0): anon0
    Twice.dfy(23,12): anon3_Else
Twice.dfy(35,32): Error: possible violation of function precondition
Execution trace:
    (0,0): anon0
    (0,0): anon9_Else
    (0,0): anon10_Then

Dafny program verifier finished with 4 verified, 2 errors