This week I reached a milestone in my most useless side project so far. I finished writing a tic-tac-toe game in Lean 4, along with proofs to guarantee that the game behaves correctly! It “only” took me 20 hours, 1000 lines of code and endless suffering… Totally worth it, as you might expect. Chances are you haven’t heard about Lean before, so I’ll share more details below. But first, let’s have an overview of this article’s contents, since it ended up quite long: