要素数nに対して、でunionをとる。
(define (union-set set1 set2) (cond ((null? set1) set2) ((null? set2) set1) (else (let ((x1 (car set1)) (x2 (car set2))) (cond ((< x1 x2) (cons x1 (union-set (cdr set1) set2))) ((= x1 x2) (cons x1 (union-set (cdr set1) (cdr set2)))) (else (cons x2 (union-set set1 (cdr set2)))))))))
(union-set '(1 2 3 4 5 6) '(0 2 3 4 5)) ;(0 1 2 3 4 5 6)