k to this definition">¶</a></dt>
<dd><p>This method registers a new listener on the signal.  It creates a
<cite>Callback</cite> object from the callback specification, attaches it to
the signal, and returns that <cite>Callback</cite> object.  When new messages
are posted to the signal, it runs the callback, passing it the
message that was posted.  If the callback itself returns a
<cite>Callback</cite> object, that indicates that the listener has started an
asynchronous operation and the poster should wait 