public SudokuGrid(SudokuGrid grid)
        {
            if (grid == null)
                throw new ArgumentNullException();

            _cells = new List<Cell>(9 * 9);

            for (int x = 0; x < 9; x++)
            {
                for (int y = 0; y < 9; y++)
                {
                    Cell cell = new Cell(this, grid.Get(x, y).Value, x, y);
                    _cells.Add(cell);
                }
            }
        }