private void OnChangeTab(object sender, SwitchPageArgs args)
        {
            current_tab     = (DocumentTab)nb_tabs.GetNthPage((int)args.PageNum);
            current_manager = current_tab.Buffer.Undoer;

            Undo.Sensitive = current_manager.CanUndo;
            Redo.Sensitive = current_manager.CanRedo;
        }
 private void AddTab()
 {
     current_tab                  = new DocumentTab(nb_tabs);
     current_manager              = current_tab.Buffer.Undoer;
     current_manager.UndoChanged += UndoChanged;
     nb_tabs.AppendPage(current_tab, current_tab.TabLabel);
     nb_tabs.ShowTabs = (nb_tabs.NPages > 1);
     nb_tabs.ShowAll();
     nb_tabs.CurrentPage = nb_tabs.PageNum(current_tab);
     nb_tabs.SwitchPage += new SwitchPageHandler(OnChangeTab);
 }