public function execute()
    {
        parent::execute();

        $xml = new DOMDocument();
        $hpsTransaction = $xml->createElement('hps:Transaction');
        $hpsCheckVoid = $xml->createElement('hps:CheckVoid');
        $hpsBlock1 = $xml->createElement('hps:Block1');

        if ($this->transactionId != null) {
            $hpsBlock1->appendChild($xml->createElement('hps:GatewayTxnId', $this->transactionId));
        } else if ($this->clientTransactionId != null) {
            $hpsBlock1->appendChild($xml->createElement('hps:ClientTxnId', $this->clientTransactionId));
        }

        $hpsCheckVoid->appendChild($hpsBlock1);
        $hpsTransaction->appendChild($hpsCheckVoid);
        return $this->service->_submitTransaction($hpsTransaction, 'CheckVoid');
    }