1. Carol will abort if she receives B == 0 (mod N) or u == 0.

2. Steve will abort if he receives A (mod N) == 0.

3. Carol must show her proof of K first. If Steve detects that Carol's proof is incorrect, he must abort without showing his own proof of K.

[1] Ok, the python code doesn't seem to, you're correct. However, that's less a demonstration of a protocol implementation, and more a demonstration of the protocol's math. The page does mention it though in the protocol section. It would be appropriate (and maybe later I'll do this) to break that out so it's more obvious.