docs: public static <E> double jensenShannonDivergence(Counter<E> c1, Counter<E> c2) {
      Counter<E> average = average(c1, c2);
      double kl1 = klDivergence(c1, average);
      double kl2 = klDivergence(c2, average);
      return (kl1 + kl2) / 2.0;
    }