Apologies if this isn't clear, I suspect if I understood the terminology better I'd already have solved the problem. I've got a foldable of a foldable of a.  (Specifically `[Set a]`) and I want to be able to express the concept that I can treat `(Foldable f1, Foldable f2) => f1 (f2 a)` as `f3 a` i.e. a foldable of a foldable of a can be newtyped to a foldable of a.  At least, I think that's right.