(define-struct pr (fst snd)) ;; pairs
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; INTERFACE Set 

;; lst->set :  [Listof X] -> [Set X]
;; create a set from the given list of objects 

;; contains? :  [Set X] X -> Boolean 
;; is elt a member of set? 

;; add-elt :  [Set X] X -> [Set X]
;; create a set from the given set and object 

;; union :  [Set X] [Set X] -> [Set X]
;; create the set that contains the elements of both 

;; intersect : [Set X] [Set X] -> [Set X]
;; create the set that contains the elements 
;; that the two sets share 

;; cartesian : [Set X] [Set Y] -> [Set (make-pr X Y)]
;; create all possible (make-pr .. ..) of elements from 
;; set1 and set2 
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; HOMEWORK for Thursday's class: 
;; Assuming the following data definition for [Set X], define
;; the functions in the interfact above.  You should attempt
;; all the functions except cartesian; we'll do cartesian in 
;; class tomorrow. 

;; A [Set X] is [List-of X]
;; where order does not matter and duplicates ARE allowed