1 ;; Exercise 3.39.  Which of the five possibilities in the parallel execution shown above remain if we instead serialize execution as follows:
2 
3 (define x 10)
4 (define s (make-serializer))
5 (parallel-execute (lambda () (set! x ((s (lambda () (* x x))))))
6 		  (s (lambda () (set! x (+ x 1)))))
7 
8 ;; 101
9 ;; 121
10 ;; 100