Geneva Ko Rule


This page describes a non-standard way of dealing with repeated positions. Most ko rules act strongly to make repeating an entire board position impossible ("super-forbidding"). Another naive approach is to still discourage plays that repeat the position, but grant players the right to, in the simplest case, recapture a ko stone. The result is that ko shapes are always resolved peacefully, with no repetitive cycling, and little of the strangeness inherent in traditional ko situations.

Rule of ko: If a move causes an exact repetition of a board position from a prior turn, the stone used to play the offending move is removed from the board (if applicable) and placed in the opponent's captures pile.[1]

In other words, repeating the (entire board) position is forbidden, in the sense of capital punishment for stones that are repeat-offenders. The tentative name "Geneva" suggests an enforced "truce" on ko fighting. The horrible crime of repeating the position is penalized, but can still be committed.

Under the Geneva Ko Rule, endlessly repeating cycles are not possible with rational players! After playing out the cycle once, the first recurring move in such a cycle would be a one stone suicide - might as well be a pass.

Next follows some reasoning that leads to adopting this approach to the ko problem, and consequences.

Intuition for b  

To avoid cycles, white b after B1 must be forbidden. But Black B1, in self-atari, has little right to assail White's position. White might expect to capture B1, and Black would normally need an approach move at b before playing inside the tiger's mouth. Similarly, if White has more external ko threats, should White have the traditional right to "win the ko" - that is, play two moves in a row in this local situation?

Result (right)  

White plays b above, capturing B1 and repeating the initial diagram. The stone W2 is forfeit. The result is Black's turn. Approaching at c next is forbidden (taboo, if you prefer).[2]


White and Black maintain a deserved amount of control over a and b respectively. The peaceful result B1 W2 is likely.


MrTenuki: Let's see what happens with kos that are more complicated:

Triple ko, Part 1 (copied from Moonshine Life)  
Triple ko, Part 2  

W6 is immediately removed after being played as it would repeat the whole-board position before B1. So...

Not a triple ko anymore  

Result: White's upper left group is dead, while the upper right corner turns into a mannen-ko.

Slarty: There is no ko for capture in the upper right. The result is seki.

Not a ko fight anymore  

If Black approaches, W2 takes, B3 takes back, clearing the ko of stones.

MrTenuki: Actually, on second thought, it seems that this proposed ko rule would turn the vast majority of kos (if not all of them) into non-kos. To see why, consider the following example (a ko for life under "normal" ko rules):


After B1, White can simply capture back with W2 at white+circle. It doesn't matter that W2 would also be removed in the process, as the important part--removing B1--is accomplished without the need of a ko threat. While Black could play a ko threat elsewhere to regain the ability to play at B1 again without losing the stone, White could always capture back. That means the White group is actually alive under the Geneva Ko Rule.

Eternal life? Not!  

For another oddity, let's look at this Eternal Life example. Given this initial position, the four-move cycle that involves B1-W2-B3 at black+circle-W4 at white+circle would be stopped at the third move (W4 can be played, but it will be removed immediately). So, the Black group is essentially alive.


[1] Use the rule in this format with territory scoring. If using area counting, add or subtract one point from komi instead.

[2] Note: If Black plays c, the position is still a repeat after the offending Black c is removed. Like passing, this is of no concern. Taking back and forth will not occur.

Geneva Ko Rule last edited by Slarty on January 4, 2012 - 05:49
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