class Board {
   List<Position> positions();
};

class InitialBoard extends Board {
   void moveTo(Position pos);

   static void startGame(GameState state);
};

class InProgressBoard extends Board {
   void moveTo(Position pos);
   void takeBack();
};

class FinishedBoard extends Board {
   void takeBack();
};


interface GameState {
   void gameWillStart(InitialBoard brd);
   void gameDidContinue(InProgressBoard brd);
   void gameDidFinishWithVictor(FinishedBoard brd, Player victor);
   void gameDidFinishWithDraw(FinishedBoard brd);

   void illegalMoveAttempted(InProgressBoard brd, Position illegalMove);
};