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

; This overwrites the old div-interval from 2.7
(define (div-interval x y)
  (let ((yl (lower-bound y))
        (yu (upper-bound y)))
    (cond ((and (<= yl 0) (>= yu 0))
             (error "division by interval including 0" y))
           (else (mul-interval x
                               (make-interval (/ 1.0 yu)
                                              (/ 1.0 yl)))))))

; 1 ]=> (div-interval (make-interval 1 3) (make-interval -1 -2))
; 
; ;Value: (-3. . -.5)
; 
; 1 ]=> (div-interval (make-interval -1 1) (make-interval -1 -2))
; 
; ;Value: (-1. . 1.)
; 
; 1 ]=> (div-interval (make-interval 1 3) (make-interval -1 1))
; 
; ;division by interval including 0 (-1 . 1)