code: def __query(self, query):
          
          # Switch to lxml for querying
          xml = self.document.toxml()
          return OneLogin_Saml2_Utils.query(fromstring(xml, forbid_dtd=True), query)
docs: Extracts a node from the DOMDocument (Logout Response Menssage)
          :param query: Xpath Expresion
          :type query: string
          :return: The queried node
          :rtype: DOMNodeList