code: def add_listener(self, listener, question):
          
          now = current_time_millis()
          self.listeners.append(listener)
          if question is not None:
              for record in self.cache.entries_with_name(question.name):
                  if question.answered_by(record) and not record.is_expired(now):
                      listener.update_record(self, now, record)
          self.notify_all()
docs: Adds a listener for a given question.  The listener will have
          its update_record method called when information is available to
          answer the question.