Maybe! You're certainly correct that there's a difference between knowing Lean (or a given proof assistant) vs. knowing mathlib (or some specific library). It d

Maybe! You're certainly correct that there's a difference between knowing Lean (or a given proof assistant) vs. knowing mathlib (or some specific library). It doesn't seem to me that Codex has much of an idea about the mathlib codebase, since it e.g. invented and used "tangent_space_at_identity" out of the blue. Library-specific training would improve on this, of course. I would reach for the usual code search tools for getting familiar with a library. For example, in Coq you have the "Search" and "Print Hint" commands which let you search for terms and instances, respectively. I imagine Lean has something similar.