static void Main(string[] args)
        {
            Document document = new Document();
            ICommand displayCmd = new DisplayCommand(document);
            ICommand redoCmd = new RedoCommand(document);
            ICommand undoCmd = new UndoCommand(document);

            Invoker invoker = new Invoker(displayCmd, redoCmd, undoCmd);
            invoker.Display();
            invoker.Redo();
            invoker.Undo();
        }
        static void Main(string[] args)
        {
            Document document   = new Document();
            ICommand displayCmd = new DisplayCommand(document);
            ICommand redoCmd    = new RedoCommand(document);
            ICommand undoCmd    = new UndoCommand(document);

            Invoker invoker = new Invoker(displayCmd, redoCmd, undoCmd);

            invoker.Display();
            invoker.Redo();
            invoker.Undo();
        }