docs: private void indexDense(DBIDRef ref, V obj) {
      double len = 0.;
      for(int dim = 0, max = obj.getDimensionality(); dim < max; dim++) {
        final double val = obj.doubleValue(dim);
        if(val == 0. || val != val) {
          continue;
        }
        len += val * val;
        getOrCreateColumn(dim).add(val, ref);
      }
      length.put(ref, FastMath.sqrt(len));
    }