docs: public static <E> Counter<E> absoluteDifference(Counter<E> c1, Counter<E> c2) {
      Counter<E> result = c1.getFactory().create();
      for (E key : Sets.union(c1.keySet(), c2.keySet())) {
        double newCount = Math.abs(c1.getCount(key) - c2.getCount(key));
        if (newCount > 0) {
          result.setCount(key, newCount);
        }
      }
      return result;
    }