type bst = Leaf | Node of int * bst * bst

let rec mem x = function
  | Leaf -> false
  | Node (y, a, b) when x < y -> mem x a
  | Node (y, a, b) when y < x -> mem x b
  | _ -> true

let rec insert x = function
  | Leaf -> Node (x, Leaf, Leaf)
  | Node (y, a, b) when x < y -> Node (y, (insert x a), b)
  | Node (y, a, b) when y < x -> Node (y, a, (insert x b))
  | Node (y, a, b) -> Node (y, a, b)