Exercise 3.11: In 3.2.3 we saw how
the environment model described the behavior of procedures with local state.
Now we have seen how internal definitions work.  A typical message-passing
procedure contains both of these aspects.  Consider the bank account procedure
of 3.1.1:


(define (make-account balance)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance 
                     (- balance 
                        amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch m)
    (cond ((eq? m 'withdraw) withdraw)
          ((eq? m 'deposit) deposit)
          (else (error "Unknown request: 
                        MAKE-ACCOUNT" 
                       m))))
  dispatch)

Show the environment structure generated by the sequence of interactions


(define acc (make-account 50))

((acc 'deposit) 40)
90

((acc 'withdraw) 60)
30


Where is the local state for acc kept?  Suppose we define another
account


(define acc2 (make-account 100))

How are the local states for the two accounts kept distinct?  Which parts of
the environment structure are shared between acc and acc2?