/// <summary>Undoes the Command operation on the Board.</summary>
/// <param name="board">Board upon which to process thic command.</param>
public void Undo(IBoardAdvanced board)
{
board.Clear(_cellIndex);
}
/// <summary>Does the Command operation on the Board.</summary>
/// <param name="board">Board upon which to process this command.</param>
public void Do(IBoardAdvanced board)
{
board.Set(_cellIndex, _digit);
}