Natural Situational Superko Rule
Natural situational superko rule: A player may not use a board play to recreate a position, if the player used a board play to create it.
The difference between the Natural situational superko rule and the situational superko rule is subtle, and is unlikely to affect any realistically played game.
The difference is that under situational superko (non-natural), you cannot make a move that creates a position that was on the board previously after you passed. With natural, you are allowed to make such a move.
Example on 5x5 board:
last point to secure territory
pass (stupid, white forgot to defend the top left corner)
White is able to retake the ko immediately at under natural situational superko. To see why, note that the resulting board position after was previously there after (a board play) and after (a pass):
- Under positional superko, is illegal because the board position is recreated from that after , even though the player to play is different.
- Under situational superko, is illegal because the board position is recreated from that after (a pass). is irrelevant since it was a different player.
- Under natural situational superko, is not illegal. The board position is recreated from that after , but because was a pass, not a board play, it's legal.
Here's a second example:
strange move, white gives black a stone needlessly
pass (no legal moves)
pass (for some weird reason)
Now White cannot capture in the corner under either situational rule, because this position was created with a board play (), not with a pass.
Santo?: I believe there is one important and not rare at all (since dead kos are very common) strategical difference between natural and situational superko. Consider any single dead ko with only one eye, like the one in moonshine life (but the position does not need to be such moonshine life at all). Under situational superko, such group is indeed dead, and the defender can at most use it for ko threats (the group might resurrect if the attacker ignores said ko threats, but that presumably means the defender has lost the corresponding ko fight and so has lost the exchange, otherwise the attacker would no have ignored). Under natural situational superko, the defender has an extra possibility: he can pass, and ignore every subsequent move by the attacker anywhere else on the board. If the attacker takes the ko, the defender is allowed to retake it immediately, since he did not create the position by playing, only by passing. Now the attacker is forced to play somewhere else or pass, and the defender can pass again, so he will never be captured. So, as soon as the group's life becomes larger than ALL of the attacker ko threats and moves on the board, the defender can make the group live by giving all of that away. This possibility does not exist under situational superko. Note that if more than one such dead ko exist, the defender will be able to save at most one, since the attacks on the other ones must be ignored if one is to save the desired group.
I believe this relatively frequent consequence of natural situational superko is "unnatural", since clear cut dead kos are not unconditionally dead anymore (They are not unconditionally dead even without any need of a moonshine life like position on the board, though with situational superko they are dead even in such positions), and the defender can always choose to save them (albeit, at the cost of allowing the attacker to play everywhere else without stop, so it would only be useful as an endgame move if the attacker has only a few ko threats whose combined total value when ignored is smaller than the group being defended value). I believe then situational superko will be considered by most to be more "natural" in it's consequences, given this considerations. On the other hand, it is true that natural situational superko is consistent in it's statement (cause and consequence of bans is the same), while situational is not. Situational might be a bit easier to state and understand by a human, though, it probably depends on the wording.
On second thought, it is probably not that common since not only ko threats,but multiple-move-attacks should be considered, and it is much rarer that ignoring every move is viable. For example, if a 5x3 or 4x4 empty region is available within a player territory, using many moves a whole live group can be created within, which would never have been possible if the defender was not ignoring every move. It still seems possible if there is a very filled board with lots of fighting, so that such possibilities are small, and the group that is involved in the dead ko is very large so saving it makes lots of sense.
The "natural" in the name of natural situational superko refers the consistency of the rule's cause and consequence. The "natural" does not refer to meeting certain traditional desires, such as killing every non-seki group with less than two eyes. Every superko rule has non-traditional strategic consequences. In theory, one can imagine (ko) rules that achieve them. For that purpose, one first needs a list of shape classes with proper definitions and their desired behaviours (precedents). Then one can design ko rules to agree to the list of precedents. Well, in principle. In practice, there has never been a complete such list of precedents. Therefore, one cannot design ko rules that are "natural" in the sense of "complying perfectly to precedents". All one can (if one wants to) achieve is agreeing to a partial list of precedents. Obviously, the first task for such a partial list is getting agreements on double ko sekis and double ko deaths: alive or dead and to be removed because of a lack of two classic eyes?
Santo?: I agree that tradition most likely cannot be achieved perfectly by simple rules. However, the differences with tradition should be rare (most people would not call the resulting game "go" otherwise, but a go variant, like Capture go).
I believe (not 100% sure though) natural situational superko gives untraditional results (dead ko lives: see my above analysis) much more often than situational or positional superko (dead kos for example exist about one in three games), both of which I believe differ only very rarely from basic ko (since the need for more than that is extremely rare).
As a matter of fact, positional and situational superko restrict the possible sequences of moves, compared to the basic ko rule. Natural situational superko restricts it even less, and for the first time for a superko rule, it allows sequences of moves which were not allowed by the basic ko rule, so it is in effect "different", and not more nor less restrictive. This extra sequences allowed can be used in about one every three games to make any desired dead ko (with one eye) live (granted, probably few of those will yield an advantage in practise, but still much more frequently than once every 5000 games or so as rare kos).
RobertJasiek: Thank you very much for pointing out this! (Positional Superko is better anyway.)