I flagged this answer as not an answer, it was disputed.
Why I believe it is not answer:
- The OP is asking for a formal proof, the answer does not provide one. Not only does it not provide one, it gives a, as far as I can see, an invalid informal argument, after all the negation of the thesis $(\neg B)\to A$ is not $(\neg B)\to \neg A$
- The answer gives an informal argument. Informal arguments often are ridiculously trivial when compared to formal proofs and this is one such case. The answer doesn't even come close of being a sufficient hint.
I'd like to know why my flag is disputed and request for the decision to be reversed.