HermanHiddema/Local global relations in rules disputes

Sub-page of HermanHiddema

In many rules disputes, there is an element where the rules make a local position seemingly illogical due to global circumstances.


Most people feel that the following white groups should be dead:

locally dead  
locally dead  

However, there are situations where the global situation make this debatable. Below are some examples.

Moonshine life

Moonshine life  

In this position, white has an unlimited number of ko threats (and so does black). So if black takes the ko in the upper right, white can always play a ko threat in the lower left and retake the ko.

This situation is known as moonshine life. The status of this situation may depend on the rule set used. Under Japanese rules, this situation is ruled as dead. Without any such special ruling, the situation is ambiguous under rule sets without superko. Under rules using superko, the situation can be played out, and there will come a point where taking any of the ko's in the lower left is not allowed due to superko restrictions.

Moonshine life & superko
start cycle  

Black starts the cycle by playing B1, intending to show that white can be killed.

White uses a ko threat, black responds...

complete cycle  

The ko threat worked, so white retakes.

Now, black makes a ko threat, and white responds...


The resulting position after W6 is the same we started with. Thus, under all superko rules, W6 is prohibited. In trying to save the upper right, white will thus lose the lower left.

Conclusion: white should pass after B1 and accept that the upper right corner is dead.

One eye flaw

With the same group, we might show the local position thus (on a 4x4 board):

locally dead  

White has just captured with white+circle, and since black has no move he can play, black passes. Under rule sets where two consecutive passes end the game, white might now pass and claim life. This is know as the 1-eye-flaw.

Note that this depends on there being no sensible legal plays, at all, for black. On a 19x19 (or even 9x9) board, this is unlikely to happen.

However, under territory scoring, if there are no more dame points, any black play would lose a point, and white might claim that black need to make such a play (and lose that point). This was the basic reasoning behind the first of the Rule Disputes Involving Go Seigen.

Unremovable threat forcing


What is the status of the marked white group?

Suppose black claims it is dead, and white challenges him to prove it.

Black tries to prove  

B1 sets out to prove that white's group can be captured.

W2 threatens to capture the upper left group

B3 takes the ko.

W4 is a ko threat. If black plays the marked point to kill white, white can retake the upper left ko, and since black has no ko threats, that group would die.

Before B1, white has no ko threats, so he couldn't play the ko in the upper left. But black has no way to remove white's option to start the ko either.

It depends on the rule set used to determine whether white's group in the lower right is alive or dead.

HermanHiddema/Local global relations in rules disputes last edited by on May 15, 2016 - 21:08
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