code: def _check_time_series(self, other):
          
          if not isinstance(other, TimeSeries):
              msg = "unsupported operand types(s) for +: %s and %s" % \
                    (type(self), type(other))
              raise TypeError(msg)
docs: Function used to check the type of the argument and raise an
          informative error message if it's not a TimeSeries.