Remember, if you hit a man over the head with a fish, he had a headache for a day. But if you teach a man to hit himself over the head with a fish, he has headaches for the rest of his life.

If your customer hunts you down and shoots you after receiving the first increment, you don't get to write future increments.

Yeah, that's correct.

Wow. Mathematical verification of requirements. Didn't we kinda get over 'proving software correct' in the late 70s?

You might find http://sauerbraten.org useful.