data BinaryTree a = Leaf | Node (BinaryTree a) a (BinaryTree a)
 deriving (Show, Eq)

insert :: (Ord a, Eq a) => a -> BinaryTree a -> BinaryTree a
insert e Leaf = Node Leaf e Leaf
insert e (Node n1 a n2)
  | e < a     = Node (insert e n1) a n2
  | otherwise = Node n1 a (insert e n2)