docs: public double[][] toMatrix(List<K1> firstKeys, List<K2> secondKeys) {
      double[][] counts = new double[firstKeys.size()][secondKeys.size()];
      for (int i = 0; i < firstKeys.size(); i++) {
        for (int j = 0; j < secondKeys.size(); j++) {
          counts[i][j] = getCount(firstKeys.get(i), secondKeys.get(j));
        }
      }
      return counts;
    }