(let ((un
       (if (cond
            ((null? '()) #f)
            (else (or (= 3 (car '())) (number-in-set? 3 (cdr '())))))
         (cons 1 (cons 2 '()))
         (cons (car (cons 3 '())) (cons 1 (cons 2 '()))))))
  (if (number-in-set? (car (cons 2 (cons 3 '()))) un)
    un
    (cons (car (cons 2 (cons 3 '()))) un)))