Exercise 3.40.  Give all possible values of x that can result from 
executing

(define x 10)

(parallel-execute (lambda () (set! x (* x x)))
                  (lambda () (set! x (* x x x))))

Which of these possibilities remain if we instead use serialized 
procedures:

(define x 10)

(define s (make-serializer))

(parallel-execute (s (lambda () (set! x (* x x))))
                  (s (lambda () (set! x (* x x x)))))

————————————————————————————————————————————————————————————————————————

Assuming that the value of x is read each time it is used in an 
expression, we have two or three reads, respectively, followed by one 
write, for each lamdba:

A: (set! x (* x x)) rA rA wA
B: (set! x (* x x x)) rB rB rB wB

It is possible that both writes happen after all the reads:

[all reads] wA wB → 1000
[all reads] wB wA → 100

It is also possible that one of the writes precedes some of the reads 
of the other procedure:

If procedure A writes 100 before some B reads:
rB rB wA rB → 10000
rB wA rB rB → 100000
wA rB rB rB → 1000000

If B writes 1000 before one or both A reads:
rA wB rA → 10000
wB rA rA → 1000000

If serialized, there are only two possibilities:

rA rA wA rB rB rB wB → 1000000
rB rB rB wB rA rA wA → 1000000