Exercise 4.18.  Consider an alternative strategy for scanning out 
definitions that translates the example in the text to

(lambda <vars>
  (let ((u '*unassigned*)
        (v '*unassigned*))
    (let ((a <e1>)
          (b <e2>))
      (set! u a)
      (set! v b))
    <e3>))

Here a and b are meant to represent new variable names, created by the 
interpreter, that do not appear in the user's program. Consider the 
solve procedure from section 3.5.4:

(define (solve f y0 dt)
  (define y (integral (delay dy) y0 dt))
  (define dy (stream-map f y))
  y)

Will this procedure work if internal definitions are scanned out as 
shown in this exercise? What if they are scanned out as shown in the 
text?  Explain.

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It will not work if scanned out as above, since the definition of dy 
refers to y, which has the value '*unassigned* at the time that 
expression is evaluated.

If scanned out as shown in the text, this will work, as the definition 
of y causes a set! y expression to be evaluated before the set! dy 
expression which uses the y value.