&rsquo;loop&rsquo;
argument is provided or if #f is provided as the &rsquo;loop&rsquo; argument
(pattern matching is used to detect the type of the third argument),
and will comprise this procedure&rsquo;s return value.  This procedure is
intended to be called within a waitable procedure invoked by a-sync
(which supplies the &rsquo;await&rsquo; and &rsquo;resume&rsquo; arguments).  It will normally
be necessary to call event-loop-block! on &rsquo;loop&rsquo; (or on the default
event loop) before invoking th