structure IntSet : SetSig =
struct
    type Item = int
    type Set  = Item list
    exception Select

    val empty = []
    fun isEmpty [] = true 
      | isEmpty  _ = false

    fun member  []      e  = false
      | member (h :: t) e  = (e = h) orelse member t e;

    fun insert(e, s)       = if member s e then s 
                             else e :: s;

    fun delete(e, [])      = [] 
      | delete(e, h::t)    = if e = h then t 
                             else h :: (delete(e, t));

    fun select []          = raise Select
      | select (h :: t)    = (h, t)

    fun union([]  , s)     = s
      | union(h::t, s)     = insert(h, union(t, s));

    fun intersect([],   s) = []
      | intersect(h::t, s) = 
          if member s h then h :: intersect(t, s)
                        else      intersect(t, s);
end;