This is a very nice introduction to theorem proving, only caveat is that you cannot save your progress and the lean interpreter in the browser can lock up.
But if you just think of it as spaced repetition I'm sure everyone will agree that it's a great feature! :)
Note that the levels are "fake", so if you lost your progress that's sad, but you can just skip ahead to where you left. (We're looking into adding localStorage or something like that... but we aren't really coders. PRs are welcome!)
But if you just think of it as spaced repetition I'm sure everyone will agree that it's a great feature! :)