 so that the event loop can only be ended by
calling event-loop-quit!, or by calling event-loop-block! again with a
#f argument (to switch the event loop back to non-blocking mode, pass
#f).  This is thread safe - any thread may call this procedure.  The
&rsquo;loop&rsquo; (event loop) argument is optional: this procedure operates on
the event loop passed in as an argument, or if none is passed (or #f
is passed as the &rsquo;loop&rsquo; argument), on the default event loop.
</p>
<p>This procedure should not