(let ((un
       (let ((un (union-nodup '() (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)))