def winner(self):
        'The winner of this board if one exists.'
        for potential_win in self._potential_wins():
            if potential_win == tuple('XXX'):
                return Outcome.win_for_crosses
            elif potential_win == tuple('OOO'):
                return Outcome.win_for_naughts
        if self._count(' ') == 0:
            return Outcome.draw
        return Outcome.ongoing
The winner of this board if one exists.