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 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.