docs: def _smooth_the_residuals(self):
          """
          Apply the MID_SPAN to the residuals of the primary smooths.
  
          "For stability reasons, it turns out to be a little better to smooth
          |r_{i}(J)| against xi" - [1]
          """
          for primary_smooth in self._primary_smooths:
              smooth = smoother.perform_smooth(self.x,
                                               primary_smooth.cross_validated_residual,
                                               MID_SPAN)
              self._residual_smooths.append(smooth.smooth_result)