Anti-seki is a quirk of the Japanese 1989 rules. The official commentary gives this example of anti-seki.
If left until the end of the game, this is an anti-seki. White's single stone in the corner is dead, because Black to play can capture it, while Black's stones are also dead, because White to play can capture them. Dead stones in an anti-seki are not removed from the board for counting.
In real games it usually occurs in the form of a double ko (anti) seki.
The commonly used Japanese rules (Nihon Ki-In 1989) define life and death as follows:
- Stones are said to be "alive" if they cannot be captured by the opponent, or if capturing them would enable a new stone to be played that the opponent could not capture. Stones which are not alive are said to be "dead."
- In the confirmation of life and death after the game stops in Article 9, recapturing in the same ko is prohibited. A player whose stone has been captured in a ko may, however, capture in that ko again after passing once for that particular ko capture.
To resolve the status of the marked black stone we have to check both conditions for life.
- Can the stone be captured? - yes, just capture it
- Does the capture enable black to play a stone that cannot be captured? - this we have to check.
is the only viable new stone black can play. It can be captured as shown in the next diagram. Therefore the marked black stone in the original diagram is not alive, and any stone that is not alive, is dead.
By similar reasoning, the marked white stone in the original diagram can be shown to be dead. All the other stones are alive, since they cannot be captured.
Here's the definition of "territory" from the same rules
Empty points surrounded by the live stones of just one player are called "eye points." Other empty points are called "dame." Stones which are alive but possess dame are said to be in "seki." Eye points surrounded by stones that are alive but not in seki are called "territory," each eye point counting as one point of territory.
Here's how to apply the rule:
The squared empty points are not surrounded by live stones of just one player (one of the surrounding stones is dead), and therefore they are not eye points. Since they are empty, they are dame. The groups with the eyes "possess dame", so they are in "seki". That means the eye points (e) they surround are not territory.
It is important to notice that the two dead stones are not in either player's territory, and that is why they are not removed when counting points.
Bill: The idea of anti-seki can also induce a player to finish a ko rather than leaving it open.
, = pass
Bill: Suppose that neither player has a ko threat and that all dame have been filled. Play stops after two passes. By the rules defining life and death both players' stones are dead and this is an antiseki. To avoid the anti-seki, White should fill the ko instead of passing.