Exercise 3.63.  Louis Reasoner asks why the sqrt-stream  procedure was 
not written in the following more straightforward way, without the local 
variable guesses:

(define (sqrt-stream x)
  (cons-stream 1.0
               (stream-map (lambda (guess)
                             (sqrt-improve guess x))
                           (sqrt-stream x))))

Alyssa P. Hacker replies that this version of the procedure is 
considerably less efficient because it performs redundant computation. 
Explain Alyssa's answer. Would the two versions still differ in 
efficiency if our implementation of delay used only (lambda () <exp>) 
without using the optimization provided by memo-proc (section 3.5.1)?

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In the efficient version, each term in the sequence is computed only 
once, using the stored value of the previous term.  In Louis's version, 
each term requires the interpreter to recalculate each previous term.

If results were not memoized the efficiency would be the same.