docs: public static final double accuracy(SummaryConfusionMatrix m) {
      final double total = m.sumOfallCells();
      double matching = 0.0;
      for (final Symbol key : Sets.intersection(m.leftLabels(), m.rightLabels())) {
        matching += m.cell(key, key);
      }
      return DoubleUtils.XOverYOrZero(matching, total);
    }