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.

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(define (upper-bound interval)
  (car interval))

(define (lower-bound intervar)
  (cdr interval))