Exercise 2.7: Alyssa’s program is incomplete because she has not specified the implementation of the interval abstraction. Here is a definition of the interval constructor:
(define (make-interval a b) (cons a b))
Define selectors upper-bound and lower-bound to complete the implementation.
(define (make-interval a b) (cons a b)) (define (lower-bound x) (car x)) (define (upper-bound x) (cdr x))