public void undo ()
	// take back the last move, ask if necessary
	{ // System.out.println("undo");
		if (Pos.haschildren() || Pos.parent() != null
			&& Pos.parent().lastchild() != Pos.parent().firstchild()
			&& Pos == Pos.parent().firstchild())
		{
			if (GF.askUndo()) doundo(Pos);
		}
		else doundo(Pos);
	}