Exercise 3.10: In the make-withdraw
procedure, the local variable balance is created as a parameter of
make-withdraw.  We could also create the local state variable
explicitly, using let, as follows:


(define (make-withdraw initial-amount)
  (let ((balance initial-amount))
    (lambda (amount)
      (if (>= balance amount)
          (begin (set! balance 
                       (- balance amount))
                 balance)
          "Insufficient funds"))))

Recall from 1.3.2 that let is simply syntactic sugar for a
procedure call:


(let ((⟨var⟩ ⟨exp⟩)) ⟨body⟩)

is interpreted as an alternate syntax for


((lambda (⟨var⟩) ⟨body⟩) ⟨exp⟩)

Use the environment model to analyze this alternate version of
make-withdraw, drawing figures like the ones above to illustrate the
interactions


(define W1 (make-withdraw 100))
(W1 50)
(define W2 (make-withdraw 100))

Show that the two versions of make-withdraw create objects with the same
behavior.  How do the environment structures differ for the two versions?