docs: def _update_variance_in_window(self):
          """
          Compute variance and covariance in window using all values in window (slow).
  
          See Also
          --------
          _add_observation_to_variances : fast update for single observation addition
          _remove_observation_from_variances : fast update for single observation removal
          """
          self._covariance_in_window = sum([(xj - self._mean_x_in_window) *
                                            (yj - self._mean_y_in_window)
                                            for xj, yj in zip(self._x_in_window, self._y_in_window)])
  
          self._variance_in_window = sum([(xj - self._mean_x_in_window) ** 2 for xj
                                          in self._x_in_window])