(define element-of-set?
  (lambda (elt set)
    (set elt))) ; test whether elt is in set
    

(define add-to-set
  (lambda (elt set)
    ; new set of elt and everything in set, i.e., {elt} U set
     (lambda (x) 
       (or (= x elt)
           (element-of-set? x set)))))

(define union-set
  (lambda (set1 set2)
    ; compute set1 U set2, i.e., set of everyting in either or both of set1 and set2
    (lambda (x)
      (or (element-of-set? x set1)
          (element-of-set? x set2)))))

(define make-singleton-set
  (lambda (elt)
    (lambda (x)
      (= elt x))))

(define make-empty-set
  (lambda ()
    (lambda (x)
      #f)))