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After events are implemented by using the  count-down timer which
will trigger the appropriate `alarm' signal (real timer -> alrm signal,
virtual timer -> vtarlm singal).

The timer is used to count down to the next dued after event(s). When the
signal is raised, at least one after event should be raised. A `current
time' is calculated 

A `current time' is maintained

After events are maintained by 3 local (non-logical) variables:

after_event_timer:  the timer used for after events (real or virtual)
after_events:       holds the event queue
next_after_time:    stores information for the next after event



After events are stored in an event queue, with each element having the
following structure:
               ev(Type,Name)-Time
where 
         Type is either single or repeat-Interval
                 single indicates a one-off, and repeat a repeated after event
                 Interval is the interval for a repeated after event
         Name is the name of the event
         Time is the time the event should be triggered.
The events are sorted by their Time (earlier first). If two events have 
the same Time, then they are in the order in which they were added (i.e. 
earlier first).

The after event queue is stored in a local variable after_events.