Lean 4 is the best codebase I've worked on. It's a functional programming language and theorem prover that really stands out. What makes it great is how it combines programming with formal proofs. The code is super clear and well-organized. Even though it's complex, it's easy to read and understand. The documentation is top-notch, with machine-checked proofs for core libraries. It has this cool metaprogramming system that lets you generate code and automate proofs elegantly. Working on Lean 4 has been amazing - it's pushed my skills to new levels. It's rare to find a project where code, math, and logic come together so nicely. Every time I dive into the codebase, I learn something new.