 with a file position pointer: there should be no need to
use this procedure with regular files, because they cannot normally
block on write and are always signalled as ready.
</p>
<p>This procedure must (like the a-sync procedure) be called in the same
thread as that in which the event loop runs.
</p>
<p>Exceptions may propagate out of this procedure if they arise while
setting up (that is, before the first call to &rsquo;await&rsquo; is made), say
because a regular file is passed to this procedure or memo