ss="pre">Clear()</span></tt></a> may be queued for
later.  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> and <a class="reference internal" href="message.html#Clear" title="Clear"><tt class="xref py py-meth docutils literal"><span class="pre">Clear()</span></tt></a>
operations will be performed in the proper order even when they
are delayed.  The <cite>callback</cite> is called when the <a class="