Gamifying propositional logic: QED, an interactive textbook
9.3(d) was a real pain until I thought about how I'd write a function with type `(Either a b, Either a c) -> Either a (b, c)`. Before that, I kept wanting a negation operator.(EDIT: Looks like some of the exercises are getting shuffled between versions -- 9.3(d) is the one I meant as of v1.2, but that might change over time. At least the type signature is a clue about which one I mean.)