Predicates.dfy(62,15): Error: assertion might not hold
Predicates.dfy(66,13): Error: assertion might not hold
Predicates.dfy(105,4): Error: a postcondition could not be proved on this return path
Predicates.dfy(104,14): Related location: this is the postcondition that could not be proved

Dafny program verifier finished with 16 verified, 3 errors
