Clean

class Foldable t
where
	fold :: (a b -> b) b (t a) -> b

instance Foldable []
where
	fold :: (a b -> b) b [a] -> b
	fold op r l = foldr op r l

instance Foldable Tree
where
	fold :: (a b -> b) b (Tree a) -> b
	fold op r Empty = r
	fold op r (Node a b c) = fold op (fold op (op b r) a) c