empty :: Set a
empty  = Empty

isEmpty :: Set a -> Bool
isEmpty Empty = True

insert :: (Ord a) => a -> Set a -> Set a
insert x Empty  = Node x Empty
insert x (Node y s)
 | x < y        = Node x (Node y s)
 | x == y       = Node y s
 | otherwise    = Node y (insert x s)

-- checks if an element is in set or not
isElem :: (Ord a) => a -> Set a -> Bool
isElem = undefined

-- removes an element from a set
delete :: (Ord a) => a -> Set a -> Set a
delete = undefined

Me falta el isElem y delete, que no consigo verlo. Gracias.