Exercise 2.8

The difference between to intervals reaches a minimum at the minuend's lower bound minus the subtrahend's upper bound. It reaches a maximum at the minuend's upper bound minus the subtrahend's lower bound.

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