def next_move(self):
        """Return a (possibly empty) list of legal moves for current player.

            Assumptions:
            The list will be empty if either player has already won, and
            the order of moves in the list is not significant

            Returns:
            A list of pairs of legal moves of the form (r, c) such that
            self._state[0][r][c] is currently unoccupied and the current player
            may move by occupying it.
        """

        move_list = []
        player = self._state[1]
        opponent = self.opponent()
        if self.winner(player):
            return move_list
        if self.winner(opponent):
            return move_list
        for i in range(len(self._state[0])):
            for b in range(len(self._state[0][i])):
                if  self._state[0][i][b]== None:
                    move_list.append((i,b))
        return move_list

    def make_move(self, move):
        """Apply move to current game.

           Arguments:
           move:  A pair (r, c) representing square self._state[0][r][c]

           Assumptions:
           (r, c) are valid coordinates for self._state[0].  If they represent
           a valid move for current player, then the current player occupies
           that position and the opponent becomes the current player.
           Otherwise self._state is left unchanged.

           Returns:
           New TicTacToe gamestate with move recorded on
           self._state[0] and current player replaced by opponent, if
           this is legal.  Otherwise return None.
        """



        if self._state[move[0]][move[1]] != None:
            return None
        else:
            board = copy.deepcopy(self._state[0])
            board[move[0]][move[1]] = self.player()
            opp = self.opponent()
            return TicTacToe((board, opp))