tfold :: b -> ( b -> a -> b -> b ) -> Tree a -> b tfold c h t = case t of Leaf -> c Node { left = l, key = k, right = r } -> h (tfold c h l) k (tfold c h r)Dann ist
leaves' = tfold ( \ fl k fr -> fl + fr ) 1 inorder' = tfold ( \ fl k fr -> fl ++ [k] ++ fr ) []