lback will execute in the event loop thread.  Actions execute in
the order in which they were posted.  If an event is posted from a
worker thread, it will normally be necessary to call event-loop-block!
beforehand.
</p>
<p>This procedure should not throw an exception unless memory is
exhausted.  If the &rsquo;action&rsquo; callback throws, and the exception is not
caught locally, it will propagate out of event-loop-run!.
</p>
<p>Where this procedure is called by other than the event loop thread,
throttling 