+-- has an identity). Instances should satisfy the following laws:
+--
+-- * @mappend mempty x = x@
+--
+-- * @mappend x mempty = x@
+--
+-- * @mappend x (mappend y z) = mappend (mappend x y) z@
+--
+-- * @mconcat = 'foldr' mappend mempty@
+--
+-- The method names refer to the monoid of lists under concatenation,