1 (define x 10)
2 (define s (make-serializer))
3 (parallel-execute (s (lambda () (set! x (* x x))))
4 		  (s (lambda () (set! x (+ x 1)))))
5 
6 (define (make-account balance)
7   (define (withdraw amount)
8     (if (>= balance amount)
9 	(begin (set! balance (- balance amount))
10 	       balance)
11 	"Insufficient funds"))
12   (define (deposit amount)
13     (set! balance (+ balance amount))
14     balance)
15   (let ((protected (make-serializer)))
16     (define (dispatch m)
17       (cond ((eq? m 'withdraw) (protected withdraw))
18 	    ((eq? m 'deposit) (protected deposit))
19 	    (else (error "Unknown request -- MAKE-ACCOUT" m))))
20     dispatch))