docs: def _add_observation_to_variances(self, xj, yj):
          """
          Quickly update the variance and co-variance for the addition of one observation.
  
          See Also
          --------
          _update_variance_in_window : compute variance considering full window
          """
          term1 = (self.window_size + 1.0) / self.window_size * (xj - self._mean_x_in_window)
          self._covariance_in_window += term1 * (yj - self._mean_y_in_window)
          self._variance_in_window += term1 * (xj - self._mean_x_in_window)