docs: protected static <O> void initializeMatrices(MatrixParadigm mat, ArrayModifiableDBIDs prots, DistanceQuery<O> dq) {
      final DBIDArrayIter ix = mat.ix, iy = mat.iy;
      final double[] distances = mat.matrix;
      int pos = 0;
      for(ix.seek(0); ix.valid(); ix.advance()) {
        for(iy.seek(0); iy.getOffset() < ix.getOffset(); iy.advance()) {
          distances[pos] = dq.distance(ix, iy);
          prots.add(iy);
          pos++;
        }
      }
      assert (prots.size() == pos);
    }