(load "2.7.txt") ; for make-interval

(define (sub-interval x y)
  (make-interval (- (lower-bound x) (upper-bound y))
                (- (upper-bound x) (lower-bound y))))

; Computes interval for -x
(define (neg-interval x)
  (make-interval (- (upper-bound x))
                 (- (lower-bound x))))

(define (sub-interval2 x y)
  (add-interval x (neg-interval y)))

; 1 ]=> (sub-interval (make-interval 1 2) (make-interval 3 4))
; 
; ;Value: (-3 . -1)
; 
; 1 ]=> (neg-interval (make-interval -4 -3))
; 
; ;Value: (3 . 4)
; 
; 1 ]=> (sub-interval2 (make-interval 1 2) (make-interval 3 4))
; 
; ;Value: (-3 . -1)