Field.dfy(12,12): Error: possible violation of function precondition
Execution trace:
    (0,0): anon0
Field.dfy(12,15): Error: assertion violation
Execution trace:
    (0,0): anon0
Field.dfy(21,12): Error: possible violation of function precondition
Execution trace:
    (0,0): anon0
Field.dfy(21,14): Error: assertion violation
Execution trace:
    (0,0): anon0

Dafny program verifier finished with 2 verified, 4 errors