Returns a DOM representation of the payment.
        @return: Element

def to_xml(self):
        '''
        Returns a DOM representation of the payment.
        @return: Element
        '''
        for n, v in { "amount": self.amount, "date": self.date,
                     "method":self.method}.items():
            if is_empty_or_none(v):
                raise PaymentError("'%s' attribute cannot be empty or " \
                                       "None." % n)

        doc = Document()
        root = doc.createElement("payment")
        super(Payment, self).to_xml(root)
        self._create_text_node(root, "amount", self.amount)
        self._create_text_node(root, "method", self.method)
        self._create_text_node(root, "reference", self.ref, True)
        self._create_text_node(root, "date", self.date)
        return root