Exercise 3.47: A semaphore (of size 
  n
) is a
generalization of a mutex.  Like a mutex, a semaphore supports acquire and
release operations, but it is more general in that up to 
  n
 processes can
acquire it concurrently.  Additional processes that attempt to acquire the
semaphore must wait for release operations.  Give implementations of semaphores


 in terms of mutexes

 in terms of atomic test-and-set! operations.