2  Union-nodup

;; union-nodup : set-of-numbers set-of-numbers  set-of-numbers
(define (union-nodup s1 s2)
  (cond
    [(null? s1) s2]
    [else (let ([un (union-nodup (cdr s1) s2)])
            (if (number-in-set? (car s1) un)
                un
                (cons (car s1) un)))]))

;; number-in-set? number set-of-numbers  boolean
(define (number-in-set? num set)
  (cond
    [(null? set) #f]
    [else (or (= num (car set))
              (number-in-set? num (cdr set)))]))

(union-nodup (cons 2 (cons 3 '())) (cons 1 (cons 2 '())))