code: def _try_next(self, get_more_allowed):
          
          if not len(self.__data) and not self.__killed and get_more_allowed:
              self._refresh()
          if len(self.__data):
              coll = self.__collection
              return coll.database._fix_outgoing(self.__data.popleft(), coll)
          else:
              return None
docs: Advance the cursor blocking for at most one getMore command.