er&rsquo; argument is
provided or if #f is provided as the &rsquo;waiter&rsquo; 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;waiter&rsquo; (or on the default
event loop) before invoking this procedure.
</p>
<p>This procedure calls &rsquo;await&rsquo; and must (