PredExpr.dfy(7,11): Error: assertion might not hold
PredExpr.dfy(39,14): Error: value does not satisfy the subset constraints of 'nat'
PredExpr.dfy(52,16): Error: assertion might not hold
PredExpr.dfy(77,13): Error: assertion might not hold

Dafny program verifier finished with 6 verified, 4 errors
