docs: protected Cluster<SubspaceModel> makeCluster(Relation<V> relation, DBIDs C, long[] D) {
      DBIDs ids = DBIDUtil.newHashSet(C); // copy, also to lose distance values!
      Cluster<SubspaceModel> cluster = new Cluster<>(ids);
      cluster.setModel(new SubspaceModel(new Subspace(D), Centroid.make(relation, ids).getArrayRef()));
      return cluster;
    }