static void Main(string[] args)
        {
            Subject   s1 = new Subject();
            IObserver o1 = new Observer1();
            IObserver o2 = new Observer2();

            s1.register(o1);
            s1.register(o2);
            s1.Flag = 5;
            s1.Flag = 25;
            s1.unregister(o1);
            s1.Flag = 50;


            /*List<string> lista1 = new List<string>() { "1", "2", "3", "4" };
             * List<string> lista2 = new List<string>() { "1", "2", "3", "6" };
             * List<string> except = lista1.exceptAll(lista2);
             *
             * except.ForEach(Console.WriteLine);
             */
            Console.ReadLine();
        }