docs: def _select_best_smooth_at_each_point(self):
          """
          Solve Eq (10) to find the best span for each observation.
  
          Stores index so we can easily grab the best residual smooth, primary smooth, etc.
          """
          for residuals_i in zip(*self._residual_smooths):
              index_of_best_span = residuals_i.index(min(residuals_i))
              self._best_span_at_each_point.append(DEFAULT_SPANS[index_of_best_span])