(let ((un
       (let ((un
              (cond
               ((null? '()) (cons 1 (cons 2 '())))
               (else
                (let ((un (union-nodup (cdr '()) (cons 1 (cons 2 '())))))
                  (if (number-in-set? (car '()) un)
                    un
                    (cons (car '()) un)))))))
         (if (number-in-set? (car (cons 3 '())) un)
           un
           (cons (car (cons 3 '())) un)))))
  (if (number-in-set? (car (cons 2 (cons 3 '()))) un)
    un
    (cons (car (cons 2 (cons 3 '()))) un)))