docs: private void indexSparse(DBIDRef ref, SparseNumberVector obj) {
      double len = 0.;
      for(int iter = obj.iter(); obj.iterValid(iter); iter = obj.iterAdvance(iter)) {
        final int dim = obj.iterDim(iter);
        final double val = obj.iterDoubleValue(iter);
        if(val == 0. || val != val) {
          continue;
        }
        len += val * val;
        getOrCreateColumn(dim).add(val, ref);
      }
      length.put(ref, len);
    }