package accessor

A legal implementation of this function must satisfy the following properties:

optional (fun a -> First (a, Fn.id)) = Fn.id

Fn.compose (optional f) (optional g)
=
optional (fun a ->
  match f a with
  | Second _ as a -> a
  | First (a, j) ->
    match g a with
    | First (a, k) -> First (a, Fn.compose j k)
    | Second a -> Second (j a))