-- > instance Foldable Tree
-- > foldMap f Empty = mempty
-- > foldMap f (Leaf x) = f x
--- > foldMap f (Node l k r) = foldMap f l `mappend` f k `mappend foldMap` f r
+-- > foldMap f (Node l k r) = foldMap f l `mappend` f k `mappend` foldMap f r
--
-- This is suitable even for abstract types, as the monoid is assumed
-- to satisfy the monoid laws.