public void Hint()
{
for (var r = 1; r <= board.Size; r++)
{
for (var c = 1; c <= board.Size; c++)
{
var s = board[r, c];
if (s.ReadOnly || s.Number != null)
{
continue;
}
s.Notes = board.Available(r, c).ToList();
}
}
}
