1 
2 Exercise 3.52.  Consider the sequence of expressions
3 
4 (define sum 0)
5 (define (accum x)
6   (set! sum (+ x sum))
7   sum)
8 (define seq (stream-map accum (stream-enumerate-interval 1 20)))
9 (define y (stream-filter even? seq))
10 (define z (stream-filter (lambda (x) (= (remainder x 5) 0))
11                          seq))
12 (stream-ref y 7)
13 (display-stream z)
14 
15 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.