SICP 問題 2.7

(define (make-interval a b) (cons a b))

(define (upper-bound x)
  (if (> (car x) (cdr x))
      (car x)
      (cdr x)))

(define (lower-bound y)
  (if (< (car y) (cdr y))
      (car y)
      (cdr y)))