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))