Exercise 3.46.  Suppose that we implement test-and-set! using an 
ordinary procedure as shown in the text, without attempting to make the 
operation atomic. Draw a timing diagram like the one in figure 3.29  to 
demonstrate how the mutex implementation can fail by allowing two 
processes to acquire the mutex at the same time.

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

At the start, the contents of the cell are false.

Process 1:                        Process 2:

test (if (car cell))              test (if (car cell))

         both see the false value and take the second branch...

(set-car! cell true)              (set-car! cell true)

         both set the cell contents to true, but it is too late, and 
         both callers of test-and-set! will now run concurrently.