Exercise 2.8. Using reasoning analogous to Alyssa’s, describe how the difference of two intervals may be computed. Define a corresponding subtraction procedure, called sub-interval.
差值的最小值是一个数的最小值减去另一个数的最大值；而差值的最大值，是一个数的最大值减去另一个数的最小值。
(define (sub-interval x y)
(make-interval (- (lower-bound x)
(upper-bound y))
(- (upper-bound x)
(lower-bound y))))
;;; tests begin
(load "../testframe.scm")
(load "7-interval-selector.scm")
(let* ((i1 (make-interval 10 20))
(i2 (make-interval 5 6))
(result (sub-interval i1 i2)))
(assertequal? (lower-bound result) (- 10 6))
(assertequal? (upper-bound result) (- 20 5)))