Exercise 3.31: The internal procedure
accept-action-procedure! defined in make-wire specifies that when
a new action procedure is added to a wire, the procedure is immediately run.
Explain why this initialization is necessary.  In particular, trace through the
half-adder example in the paragraphs above and say how the system’s response
would differ if we had defined accept-action-procedure! as


(define (accept-action-procedure! proc)
  (set! action-procedures 
        (cons proc action-procedures)))