private RowUniquenessRule(RowUniquenessRule existing, IReadOnlyPuzzle puzzle)
{
_puzzle = puzzle;
_unsetRowValues = existing._unsetRowValues.AsSpan().ToArray();
}
private RowUniquenessRule(RowUniquenessRule existing, IReadOnlyPuzzle?puzzle)
: base(existing)
{
_puzzle = puzzle;
}