#lang sicp

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

(define (lower-bound i)
  (car i))

(define (upper-bound i)
  (cdr i))

(define (sub-interval a b)
  (make-interval (- (lower-bound a)
                    (upper-bound b))
                 (- (upper-bound a)
                    (lower-bound b))))