public SudokuPuzzle Clone()
{
var puzzle = new SudokuPuzzle();
for (var i = 0; i < 9; i++)
{
for (var j = 0; j < 9; j++)
{
Cell cell;
if ((cell = GetCell(i, j)).Value != null)
{
puzzle.SetCellValue(i, j, cell.Value.Value);
}
}
}
return(puzzle);
}