docs: public MeanVariance averageSymmetricGini() {
      MeanVariance mv = new MeanVariance();
      for(int i1 = 0; i1 < size1; i1++) {
        double purity = 0.0;
        if(contingency[i1][size2] > 0) {
          final double cs = contingency[i1][size2]; // sum, as double.
          for(int i2 = 0; i2 < size2; i2++) {
            double rel = contingency[i1][i2] / cs;
            purity += rel * rel;
          }
          mv.put(purity, cs);
        }
      }
      for(int i2 = 0; i2 < size2; i2++) {
        double purity = 0.0;
        if(contingency[size1][i2] > 0) {
          final double cs = contingency[size1][i2]; // sum, as double.
          for(int i1 = 0; i1 < size1; i1++) {
            double rel = contingency[i1][i2] / cs;
            purity += rel * rel;
          }
          mv.put(purity, cs);
        }
      }
      return mv;
    }