Superko rules prohibit recurrences of an earlier board position under certain circumstances. The recurrence of such positions could be caused by long cycles, such as triple ko, eternal life or round-robin ko.
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Superko rules come in various forms, depending on when they forbid recreating a board position; factors include:
- on whose turn the position was previously created;
- whether players have passed, immediately before or at some intervening time;
- the type of move recreating the position (see Chinese Superko below).
The main variants of the superko rule in use are:
- Positional Superko: Forbids a board play to repeat a board position. See Positional Superko / Example.
- Chinese Superko: A play may not recreate a previous board position by means of basic ko or sending two, returning one.
- This, according to the sixth meeting of the International Go Rules Forum, is what the Chinese rules were intended to specify.
- Situational Superko: Forbids a board play to repeat a board position with the same player to move, regardless whether earlier it was created by a board play or pass play.
- Natural Situational Superko: Forbids a player’s board play to repeat a board position that they created with an earlier board play.
- Note that it could allow an immediate ko recapture if the first capture was preceded by a pass.
- BGA Rules 2009 use this type of superko but the basic ko rule as well, forbidding the immediate ko recapture mentioned above.
- According to Terry Benson, AGA rules were meant to use Natural Situational Superko, but everybody else interprets them with Situational Superko.
The following variants have been used in theoretical discussions:
- Strict Superko Term suggested by Robert Pauli: forbids “completing a non-trivial cycle” (in his terminology).
- Sound Superko Term suggested by Robert Pauli: forbids “completing a cycle” (in his terminology) that does not include a pass.
The most commonly used example why a superko rule is necessary is triple ko. See below for a more simple case that also illustrates the rule:
This situation is a seki in the corner. Assume Black just passed, so that this position has occurred with either side to play. If White plays at a, Black plays b. With a superko rule in place, White cannot retake at , so Black can take the corner. Of course, this comes from a White initial mistake, but see Rules Beast 1 for a case where White breaks the seki and wins the fight...
- Pinwheel Ko — A network of kos rotating round the board
- Rules Beast 1 — A position where superko surprisingly requires an extra move in a seki to avoid a capture
- Superko Puzzle 2 — In which an unconditionally live group is sacrificed to break a cycle
- Fixed ko rule: repeated positions may occur but you are not allowed to make the same move in them.
- Long cycle rule: Rather than forbidding the repetition a board position, this rule assigns a result to the game if a repetition occurs. The game result depends on the number of stones captured by either since the first occurrence of the position.
- The Computer Olympiad rules use this version.
- Short cycle rules?: The result depends on the number of moves between the repeats. If there are 3 or fewer moves between the repeats the game is a loss for the player who makes the repeat. If there are more moves between the repeats the game is a draw.
- Long cycle seki rule: The groups involved in a long cycle are treated as alive in seki
- No result: The Japanese rules and Korean rules say that if a board position is repeated, the game has no result; this usually means the game must be replayed.
- Colour-blind superko: Ignore the colours, suggested as a ko-rule for multi-colour go.
- Quadruple Ko Fight – a superko experiment
- http://home.snafu.de/jasiek/superko.html – lists five variants and the advantages and disadvantages of the major three (positional / situational / natural situational)
- Article 7: The repetition of the same pattern shall be prohibited unless the right to an alternate move is disregarded.
In modern terms, "disregarding the right to an alternate play" means passing. When a pass lifts a ko or superko ban, this is sometimes called "pass for ko threat". There are different versions of the superko rule in which an intervening pass allows a repetition. Yasunaga's seems to say that any intervening pass lifts the restriction on repetition, but it may mean a pass by the player who makes the repetition. The interaction with passes can pose problems with ending the game, but not taking passes into account can lead to strange results. My home page, when I get it up, will have a discussion, and my solution to the problem, based on Combinatorial Game Theory. Meanwhile, for my solution, see Spight Rules.
Yasunaga proposed a three-consecutive-pass rule, which is how my solution will usually work. Ing uses a four-pass rule.