public function add(Horde_Imap_Client_Interaction_Command $cmd,
                        $top = false)
    {
        if ($top) {
            // This won't re-index keys, which may be numerical.
            $this->_commands = array($cmd->tag => $cmd) + $this->_commands;
        } else {
            $this->_commands[$cmd->tag] = $cmd;
        }
        $this->_todo[$cmd->tag] = true;
    }