-- A commonly seen type class used for
-- combining things. 
class MyMonoid a where
  mempty' :: a
  mappend' :: a -> a -> a

instance MyMonoid Bool where
  mempty' = True
  mappend' x y = x && y

instance MyMonoid Integer where
  mempty' = 0 
  mappend' x y = x + y

instance (MyMonoid a) => MyMonoid (Maybe a) where
  mempty' = Nothing
  mappend' (Nothing) (Just x) = Just x
  mappend' (Just y) (Just x) = Just (mappend' y x)
  mappend' (Just y) (Nothing) = Just y

instance MyMonoid [a] where
  mempty' = []
  mappend' = (++)