ded until the callback is exectured.  In this way, the
<a class="reference internal" href="#Post" title="Post"><tt class="xref py py-meth docutils literal"><span class="pre">Post()</span></tt></a> call can operate asynchronously, and so the
<cite>callback</cite> parameter is used to synchronize with its operation;
the <cite>callback</cite> will be called when all the listeners have responded
to the post.</p>
<p>If a <a class="reference internal" href="#Post" title="Post"><tt class="xref py py-meth docutils 