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

  variant ~match_:Either.first ~construct:Fn.id = Fn.id

  Fn.compose
    (variant ~match_:m1 ~construct:c1)
    (variant ~match_:m2 ~construct:c2)
  =
  variant
    ~match_:(fun a ->
      match m1 a with
      | Second _ as a -> a
      | First a ->
        match m2 a with
        | First _ as a -> a
        | Second a -> Second (c1 a))
    ~construct:(Fn.compose c1 c2)