calc.dfy(8,16): Error BP5003: A postcondition might not hold on this return path.
calc.dfy(8,10): Related location: This is the postcondition that might not hold.
Execution trace:
    (0,0): anon0
    calc.dfy(9,5): anon2_Else
calc.dfy(15,16): Error BP5003: A postcondition might not hold on this return path.
calc.dfy(15,10): Related location: This is the postcondition that might not hold.
Execution trace:
    (0,0): anon0

Dafny program verifier finished with 2 verified, 2 errors