 procedure invoked by a-sync (which
supplies the &rsquo;resume&rsquo; argument).  The watch is multi-shot - it is for
the user to bring it to an end at the right time by calling
event-loop-remove-write-watch! in the waitable procedure.  This
procedure is mainly intended as something from which higher-level
asynchronous file operations can be constructed.  The &rsquo;loop&rsquo; argument
is optional: this procedure operates on the event loop passed in as an
argument, or if none is passed (or #f is passed), o