👍 LGTM

I wondered if there was some trick we could use here to avoid the need for two versions of V, so you could choose which strategy (semiring or semigroup) to use via newtype or something, but I don't know if it's worth it?

I'd be happy to unify the two. I think a solution is to use the Semiring version and make a type synonym (and helper functions) in another module for V (Additive s).

👍