Exercise 3.32.  The procedures to be run during each time segment of the 
agenda are kept in a queue. Thus, the procedures for each segment are 
called in the order in which they were added to the agenda (first in, 
first out). Explain why this order must be used. In particular, trace 
the behavior of an and-gate whose inputs change from 0,1 to 1,0 in the 
same segment and say how the behavior would differ if we stored a 
segment's procedures in an ordinary list, adding and removing procedures 
only at the front (last in, first out).

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The and-gate definition given in the text:

(define (and-gate a1 a2 output)
  (define (and-action-procedure)
    (let ((new-value
           (logical-and (get-signal a1) (get-signal a2))))
      (after-delay and-gate-delay
                   (lambda ()
                     (set-signal! output new-value)))))
  (add-action! a1 and-action-procedure)
  (add-action! a2 and-action-procedure)
  'ok)

In a change from 0,1 to 1,0, both the a1 and a2 input values are change 
and so the and-action-procedure will be called twice.

If the a1 signal is changed first, then in the first call to after-delay 
the values will be 1,1, since the a1 signal has changed and the a2 
signal has not yet changed, and so the output signal will be 1.  If the 
a2 signal changes next, the correct output 0 will be added to the agenda 
after the incorrect output.  Since spurious transient outputs may be 
added to the agenda before correct ones, it is important that the last 
item added to the agenda at a given time segment is the last one called.