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)))

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This is necessary to meet the programmer's expectations of how such a 
system should work by analogy with a real circuit.

Since action procedures are only called when the value on an input wire 
changes, many changes would not fully propagate through larger circuits 
even after all of the inputs had changed once.

For example, creating a half-adder circuit and then setting the A input 
to 1 would not change the output of the inverter (E in figure 3.25), 
since the input to the inverter would still be 0.

To fully 'initialize' such a model, it would be necessary to exercise 
every possible combination of inputs to each component before correct 
output would be assured.