Append a :class:`Payment <stellar_base.operation.Payment>` operation
        to the list of operations.

        :param str destination: Account address that receives the payment.
        :param str amount: The amount of the currency to send in the payment.
        :param str asset_code: The asset code for the asset to send.
        :param asset_issuer: The address of the issuer of the asset.
        :type asset_issuer: str, None
        :param str source: The source address of the payment.
        :return: This builder instance.

def append_payment_op(self,
                          destination,
                          amount,
                          asset_code='XLM',
                          asset_issuer=None,
                          source=None):
        """Append a :class:`Payment <stellar_base.operation.Payment>` operation
        to the list of operations.

        :param str destination: Account address that receives the payment.
        :param str amount: The amount of the currency to send in the payment.
        :param str asset_code: The asset code for the asset to send.
        :param asset_issuer: The address of the issuer of the asset.
        :type asset_issuer: str, None
        :param str source: The source address of the payment.
        :return: This builder instance.

        """
        asset = Asset(code=asset_code, issuer=asset_issuer)
        op = operation.Payment(destination, asset, amount, source)
        return self.append_op(op)