public boolean set( int row, int column, int value ){
        boolean flag = (0 <= row && row < DIGITS &&
                        0 <= column && column < DIGITS &&
                        0 < value && value <= DIGITS &&
                        board[row][column] == 0);
        if( flag ){
            board[row][column] = value;
            flag = valid();
            if( !flag ){ board[row][column] = 0; }
        }
        return flag;
    }