(*

   SET of Int 
 
   New:    --> SET
   Insert: SET x int --> SET
   remove: SET x int --> SET
   member: SET x int --> boolean

*)


datatype IntSet = 
   New  
   | Insert of IntSet * int ;

fun member ( New, i )         = false 
  | member ( Insert(s,j), i ) = if i=j 
                                 then true 
                                 else member(s,i) ;

fun remove ( New, i )         = New
  | remove ( Insert(s,j), i ) = if i=j 
                                 then remove(s,i)
                                 else Insert( remove(s,i), j) ;
                                  

(*

fun eq ( New, New ) = true
  | eq ( New, Insert(S,i) ) = false
  | eq ( Insert(S,i), New ) = false
  | eq ( Insert(S,i), Insert(R,j) ) =
      if (i=j) then eq(remove(S,i),remove(R,j))
      else (* more cases here *)

*)

(* 
  also try adding a function "size" to tell you how many elements
  have been added to a set
   size: SET   --> int
*)