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Each instance of this class manages its own queue.
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supposed to return the current time, one that is supposed to
implement a delay.  You can implement real-time scheduling by
substituting time and sleep from built-in module time, or you can
implement simulated time by writing your own functions.  This can
also be used to integrate scheduling with STDWIN events; the delay
function is allowed to modify the queue.  Time can be expressed as
integers or floating point numbers, as long as it is consistent.

Events are specified by tuples (time, priority, action, argument).
As in UNIX, lower priority numbers mean higher priority; in this
way the queue can be maintained as a priority queue.  Execution of the
event means calling the action function, passing it the argument
sequence in "argument" (remember that in Python, multiple function
arguments are be packed in a sequence).
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        restarted.

        It is legal for both the delay function and the action
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        exceptions are not caught but the scheduler's state remains
        well-defined so run() may be called again.

        A questionable hack is added to allow other threads to run:
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