The signs of the endpoints of the intervals

In passing, Ben also cryptically comments: «By testing the signs of the endpoints of the intervals, it is possible to break mul-interval into nine cases, only one of which requires more than two multiplications». Rewrite this procedure using Ben's suggestion.

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

(define (lower-bound interval)
  (car interval))

(define (upper-bound interval)
  (cdr interval))

(define interval (make-interval -5 15))
(define interval2 (make-interval 5 10))
(define interval3 (make-interval 6 8))

(define result1 (mul-interval interval interval2))
(define result2 (mul-interval interval2 interval))
(define result3 (mul-interval interval2 interval3))

(check-equal? result1 result2)
(check-equal? result3 '(30 . 80))