(* Represent a set as a binary search tree (BST). 
   This is a binary tree in which for every node Node(l,v,r),
     all values in l are < v and all values in r are > v. *)

module BSTSet : SET = struct

  open Bintree 

  type 'a set = 'a bintree

  let empty = Leaf
  
  let singleton x = Node(Leaf, x, Leaf)

  let rec insert x s = Leaf (* Replace this stub *)

  let rec delete x s = Leaf (* Replace this stub *)

  let rec member x s = false (* Replace this stub *)

  let rec toList s = [] (* Replace this stub *)

  let rec union s1 s2 = Leaf (* Replace this stub *)

  let rec intersection s1 s2 = Leaf (* Replace this stub *)

  let rec difference s1 s2 = Leaf (* Replace this stub *)

  let rec fromList xs = Leaf (* Replace this stub *)

  let rec toSexp eltToSexp s = Sexp.Seq []

  let rec fromSexp eltFromSexp sexp = empty 

  let toString eltToString s = "*" (* Replace this stub *)

end