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Triple Ko
  Difficulty: Beginner   Keywords: Ko

In a Triple Ko, there are three independent Kos on the board.

Say you have the kos `A', `B' and `C'. You take `A', your opponent takes `B', you take `C'. Your opponent retakes `A', you retake `B' and he retakes `C'. With the simple ko rule, you could now retake `A' again, and the cycle would go on for ever.

Ofcourse, this is undesirable. Therefore, several rulesets incorporate a so-called `superko' rule, which says that you are not allowed to reproduce a previous position. In the case above, if you were to retake `A', you would break the rule and forfeit the game.

Specifically, there is the so-called positional superko rule, in which case it is the position which cannot be repeated, or the situational superko rule, in which case it is not just the position, but also the situation (i.e. the player to play) which cannot be repeated.

In the A/B/C ko example above, under the situational superko rule, it is you who would break the rule if you captured A again, after one cycle. Under the positional superko rule, it would be your opponent who breaks the rule when he captures A (halfway through the cycle).

Some Ko-rules allow one repetition, but not more.

The superko rule also has other implications; normally the board position repeats after a few moves (in the above example, after six moves). However, the sequences of moves, which produce the same board position, can be large and involve captures by both sides. As long as both parties are unwilling to play another line of play (they both consider the play which leads to the 'superko' optimal) then the superko situation will arise.

It should be noted that, under current Japanese rules, situations which would be prohibited by a superko rule will lead to a `no-result', and not to a loss for the player who plays the move.

However, the positions are very rare. I have never seen one in real life. When they do occur, they are normally talked a lot about - because they are so rare.


[Diagram]
Diag.: An example of triple ko

If either of the sides is leading enough to discard a group here then the game may be continued. Otherwise, if the winner is determined by who kills here, they have to fight 3 ko. Black captures at 'a', white at 'b', black at 'c' and then white has to capture at 'a' if rules allow a return to the initial position.

 HolIgor

For more information about the various rulesets in general and korules in particular, take a look at [ext] Robert JASIEKs homepage

--MortenPahle



This is a copy of the living page "Triple Ko" at Sensei's Library.
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