code: void set_sectioned_gain()
      {
          double gain = 0.0f;
          if (sectioned_gain_ == 0 && sectioned_clusters_.size() > 1)
          {
              for (Cluster<K> cluster : sectioned_clusters_)
              {
                  gain += cluster.composite_vector().norm();
              }
              gain -= composite_.norm();
          }
          sectioned_gain_ = gain;
      }
docs: Set a gain when the cluster sectioned.