MonoidTwoMappend.hs

data Two a b =
  Two a
      b
  deriving (Eq, Show)

instance (Semigroup a, Semigroup b) =>
         Semigroup (Two a b) where
  (Two x1 y1) <> (Two x2 y2) = Two (x1 M.<> x2) (y1 M.<> y2)

instance (Monoid a, Semigroup a, Monoid b, Semigroup b) =>
         Monoid (Two a b) where
  mempty = Two mempty mempty
  mappend = (S.<>)