if (to_index < 0)
return;
var pages = args.SelectionData.Data as Hyena.Gui.DragDropList<Page>;
Hyena.Gui.DragDropList<Page> pages = args.SelectionData;
to_index -= pages.Count (p => p.Index < to_index);
var action = new MoveAction (document, pages, to_index);
action.Do ();