It seems like sound testing methodology to identify important theorems related to the code, prove them, and then verify the proof.

Verification gets sold as "bulletproof" but I'm skeptical for a couple reasons:

- How do you establish the relationship between the code and the theorem? Lean theorem can be applied to zlib implemented in Lean, what if you want to check zlib implemented in a normal programming language like C, JS, Zig, or whatever?

- How do you know the key properties mean what you think they mean? E.g. the theorem says "ZlibDecode.decompressSingle (ZlibEncode.compress data level) = .ok data" but it feels like it would be very easy to accidentally prove ∃ x s.t. decompress(compress(x)) == x while thinking you proved ∀ x, decompress(compress(x)) == x.

I've tried Lean and Coq and...I don't really like them. The proofs use specialized programming languages. And they seem deliberately designed to require you to use a context explorer to have any hope of understanding the proof at all. OTOH a normal unit test is written in a general purpose programming language (usually the same one as the program being tested), I'm much more comfortable checking that a Claude-written unit test does what I think it's doing than a Claude-written Lean proof of correctness.