Accept-action-procedure! procedure

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 accept-action-procedure! as

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

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