1 (define (exchange account1 account2)
2   (let ((difference (- (account1 'balance)
3 		       (account2 'balance))))
4     ((account1 'withdraw) difference)
5     ((account2 'deposit) difference)))
6 
7 (define (make-account-and-serializer balance)
8   (define (withdraw amount)
9     (if (>= balance amount)
10 	(begin (set! balance (- balance amount))
11 	       balance)
12 	"Insufficient funds"))
13   (define (deposit amount)
14     (set! balance (+ balance amount))
15     balance)
16   (let ((balance-serializer (make-serializer)))
17     (define (dispatch m)
18       (cond ((eq? m 'withdraw) withdraw)
19 	    ((eq? m 'deposit) deposit)
20 	    ((eq? m 'balance) balance)
21 	    ((eq? m 'serializer) balance-serializer)
22 	    (else (error "Unknown request -- MAKE-ACCOUT"
23 			 m))))
24     dispatch))
25 
26 (define (deposit account amount)
27   (let ((s (account 'serializer))
28 	(d (account 'deposit)))
29     ((s d) amount)))
30 
31 (define (serialized-exchange account1 account2)
32   (let ((serializer1 (account1 'serializer))
33 	(serializer2 (account2 'serializer)))
34     ((serializer1 (serializer2 exchange))
35      account1
36      account2)))