연습문제 2.62

(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) (cdr set2))))
                     ((< x1 x2) (cons x1 (union-set (cdr set1) set2)))
                     (else (cons x2 (union-set set1 (cdr x2)))))))))