docs: public static <T1, T2> Counter<T2> transform(Counter<T1> c, Function<T1, T2> f) {
      Counter<T2> c2 = new ClassicCounter<T2>();
      for (T1 key : c.keySet()) {
        c2.setCount(f.apply(key), c.getCount(key));
      }
      return c2;
    }