docs: public static <E> Counter<E> average(Counter<E> c1, Counter<E> c2) {
      Counter<E> average = c1.getFactory().create();
      Set<E> allKeys = new HashSet<E>(c1.keySet());
      allKeys.addAll(c2.keySet());
      for (E key : allKeys) {
        average.setCount(key, (c1.getCount(key) + c2.getCount(key)) * 0.5);
      }
      return average;
    }