code: public static <E> Counter<E> division(Counter<E> c1, Counter<E> c2) {
      Counter<E> result = c1.getFactory().create();
      for (E key : Sets.union(c1.keySet(), c2.keySet())) {
        result.setCount(key, c1.getCount(key) / c2.getCount(key));
      }
      return result;
    }
docs: Returns c1 divided by c2. Note that this can create NaN if c1 has non-zero
  counts for keys that c2 has zero counts.
  
  @return c1 divided by c2.