docs: private ArrayList<ArrayDBIDs> buildOneDimIndexes(Relation<? extends NumberVector> relation) {
      final int dim = RelationUtil.dimensionality(relation);
      ArrayList<ArrayDBIDs> subspaceIndex = new ArrayList<>(dim + 1);
  
      SortDBIDsBySingleDimension comp = new VectorUtil.SortDBIDsBySingleDimension(relation);
      for(int i = 0; i < dim; i++) {
        ArrayModifiableDBIDs amDBIDs = DBIDUtil.newArray(relation.getDBIDs());
        comp.setDimension(i);
        amDBIDs.sort(comp);
        subspaceIndex.add(amDBIDs);
      }
  
      return subspaceIndex;
    }