(let ((un
       (cond
        (else
         (let ((un (union-nodup (cdr (cons 3 '())) (cons 1 (cons 2 '())))))
           (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)))