The value of sum

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.

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