Exercise 3.52: Consider the sequence of
expressions


(define sum 0)

(define (accum x)
  (set! sum (+ x sum))
  sum)

(define seq 
  (stream-map 
   accum 
   (stream-enumerate-interval 1 20)))

(define y (stream-filter even? seq))

(define z 
  (stream-filter 
   (lambda (x) 
     (= (remainder x 5) 0)) seq))

(stream-ref y 7)
(display-stream z)

What is the value of sum after each of the above expressions is
evaluated?  What is the printed response to evaluating the stream-ref
and display-stream expressions?  Would these responses differ if we had
implemented (delay ⟨exp⟩) simply as (lambda () ⟨exp⟩)
without using the optimization provided by memo-proc?  Explain.