con-nf VS lean4checker

But like, you can look at what parts of Mathlib this development imports, it's mainly stuff imported by files in this subdirectory https://github.com/leanprover-community/con-nf/tree/main/Con... , and it's pretty basic things: the definition of a permutation, a cardinal number etc. Almost all of these are things that would feature in the first one or two years of an undergraduate math degree (from just quickly scanning it, the most advanced thing I could see is the definition of cofinality of ordinals). It seems practically impossible to me that someone would make a mistake when e.g. defining what a group is, in a way subtle enough to later break this advanced theorem. If you think that people could mess up that, then all of math would be in doubt.

Note also that there is an independent checker https://github.com/leanprover/lean4checker to ensure that you're not pulling any fancy tricks at the code level: that the compiled output, free of tactics, is in fact a proof.