2.08¶

; Using reasoning analogous to Alyssa’s, describe how the 
; difference of two intervals may be computed. Define a 
; corresponding subtraction procedure, called `sub-interval`.

(define (make-interval a b) (cons a b))
(define (add-interval x y)
    (make-interval (+ (lower-bound x) (lower-bound y))
                   (+ (upper-bound x) (upper-bound y))))

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