public function sendSetGroupSubject($gjid, $subject)
    {
        $child = new ProtocolNode('subject', null, null, $subject);
        $node = new ProtocolNode('iq',
            [
                'id'    => $this->createIqId(),
                'type'  => 'set',
                'to'    => $this->getJID($gjid),
                'xmlns' => 'w:g2',
            ], [$child], null);

        $this->sendNode($node);
    }