`type Extension s = s -> Endo Result` is already good enough IMO, since it's `Monoid` and we can also contramap thanks to `Profunctor (->)`. We don't have to br

`type Extension s = s -> Endo Result` is already good enough IMO, since it's `Monoid` and we can also contramap thanks to `Profunctor (->)`. We don't have to bring in `ApAp`, which is anyway called `Compose` in `Data.Functor.Compose`.