public static void main(String[] args) {
        int size = 10;

        Set s1 = new HashSet<Holder<Integer>>(size);    // a
        for (int i = 0; i < size; ++i) {
            s1.add(new Holder<Integer>(i));
        }                                // b

        System.out.println("Set 1 size: " + s1.size());// c
        System.out.println("Set 1 itself: " + s1);// d

        Set s2 = new HashSet<Holder<Integer>>(size); //e
        for (int i = 0; i < size; ++i) {
            s2.add(new Holder<Integer>(i + 1));
        }
        System.out.println("Set 2 size: " + s2.size());
        System.out.println("Set 2 itself: " + s2);


        s1.addAll(s2);

        System.out.println("United set size: " + s1.size());
        System.out.println("United set itself: " + s1);

    }