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

        HpsInputValidation::checkCurrency($this->currency);
        HpsInputValidation::checkAmount($this->amount);

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

        $hpsBlock1->appendChild($xml->createElement('hps:Amt', $this->amount));

        if ($this->authAmount != null) {
            $hpsBlock1->appendChild($xml->createElement('hps:AuthAmt', $this->authAmount));
        }

        if ($this->card != null) {
            $cardData = $xml->createElement('hps:CardData');
            $cardData->appendChild($this->service->_hydrateManualEntry(
                $this->card,
                $xml
            ));
            $hpsBlock1->appendChild($cardData);
        } else if ($this->token != null) {
            $cardData = $xml->createElement('hps:CardData');
            $cardData->appendChild($this->service->_hydrateTokenData(
                $this->token,
                $xml
            ));
            $hpsBlock1->appendChild($cardData);
        } else {
            $hpsBlock1->appendChild($xml->createElement('hps:GatewayTxnId', $this->transactionId));
        }

        if ($this->details != null) {
            $hpsBlock1->appendChild($this->service->_hydrateAdditionalTxnFields($this->details, $xml));
        }

        $hpsCreditReversal->appendChild($hpsBlock1);
        $hpsTransaction->appendChild($hpsCreditReversal);

        return $this->service->_submitTransaction($hpsTransaction, 'CreditReversal', (isset($this->details->clientTransactionId) ? $this->details->clientTransationId : null));
    }