(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)))