Life In 19x19
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rule discussion
http://www.lifein19x19.com/viewtopic.php?f=45&t=16620
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Author:  macelee [ Sat May 11, 2019 11:44 am ]
Post subject:  rule discussion

This is being circulated in the Chinese Go community now. The game as shown below is taken from Golaxy's training data.

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This is a very special situation. The upper-left group is a 'bent four in the corner' so white is normally dead. Black would fix all the ko threats elsewhere before starting the ko at A18. Black would then take the ko first so white is hopeless.

However in this game are the two black stones at K15 and K16. White can take the two stones by playing L15 which is an absolute sente move. That means black would never be able to fix all the ko threats. So black cannot start the ko at A18.

But does that mean the two black stones, which are normally considered dead, are actually alive in this game?

Interesting challenge for many rule sets.

Author:  Knotwilg [ Sat May 11, 2019 12:48 pm ]
Post subject:  Re: rule discussion

Under any rule set where presumably dead stones can be captured so as to be removed without penalty, this situation will stand out as one where removing the stones comes with a penalty, so these two black stones are alive. Their life and death is tightly coupled with that of the stones in the corner.

Only in rule sets where removal of presmably dead stones by play is discouraged, ambiguity remains and must be resolved by particular call outs.

Author:  Bill Spight [ Sat May 11, 2019 3:37 pm ]
Post subject:  Re: rule discussion

macelee wrote:
This is being circulated in the Chinese Go community now. The game as shown below is taken from Golaxy's training data.

Attachment:
70e34ff0fc05a6a3f3feac0fc3c24bb8.png


This is a very special situation. The upper-left group is a 'bent four in the corner' so white is normally dead. Black would fix all the ko threats elsewhere before starting the ko at A18. Black would then take the ko first so white is hopeless.

However in this game are the two black stones at K15 and K16. White can take the two stones by playing L15 which is an absolute sente move. That means black would never be able to fix all the ko threats. So black cannot start the ko at A18.

But does that mean the two black stones, which are normally considered dead, are actually alive in this game?


Under AGA, Ing, and, IIUC, Chinese rules, the stones are alive. White has an unremovable ko threat, unless she captures them, in which case the Bent Four corner dies. So they are invulnerable. Under Japanese rules the Bent Four corner dies anyway, so White can and should capture the stones before play ends. I don't know about Korean rules.

Quote:
Interesting challenge for many rule sets.


Ed Lee and I discussed a similar position, probably inspired by this one, starting with this post: viewtopic.php?p=243731#p243731

I am unsure about ancient Chinese territory scoring with a group tax. ;)

Author:  EdLee [ Sat May 11, 2019 7:45 pm ]
Post subject: 

Hi Bill,

Yes, you guessed correctly. :tmbup:

Author:  sorin [ Sat May 11, 2019 9:12 pm ]
Post subject:  Re: rule discussion

I was under the wrong impression that bent-four is automatically dead in the Japanese rules...

I had no idea about "unremovable ko threats"; maybe never too late to learn the rules of this game :-)

Author:  jlt [ Sat May 11, 2019 11:23 pm ]
Post subject:  Re: rule discussion

Bill Spight wrote:
Under Japanese the Bent Four corner dies anyway

sorin wrote:
I was under the wrong impression that bent-four is automatically dead in the Japanese rules...


These two statements seem to contradict each other.

Author:  sorin [ Sun May 12, 2019 11:49 am ]
Post subject:  Re: rule discussion

jlt wrote:
Bill Spight wrote:
Under Japanese the Bent Four corner dies anyway

sorin wrote:
I was under the wrong impression that bent-four is automatically dead in the Japanese rules...


These two statements seem to contradict each other.


Ah, just when I thought I learned the rules...
At least I learned about unremovable threats.

I think I got confused by the mention of unremovable ko threats at https://senseis.xmp.net/?BentFourInTheCornerIsDead
So under Japanese rules, it is always dead. Except when it isn't: if the surrounding group doesn't have two eyes, the ko has to be played.

Japanese rules are so unnecessarily complicated :-)

Author:  Bojanic [ Thu May 16, 2019 2:10 am ]
Post subject:  Re: rule discussion

sorin wrote:
I was under the wrong impression that bent-four is automatically dead in the Japanese rules...

I had no idea about "unremovable ko threats"; maybe never too late to learn the rules of this game :-)

I think that there is no rule that bent-four is automatically dead.
It was just explanation that you can capture it by removing all ko threats first, then starting ko.
And if there is "unremovable ko threat", then you can not remove it first.

But seriously, what are the odds of such situation happening in the game?
Bent four is rare, such seki even rarer, combination of two rare*rarer.
Answer could lead to explanation of why is Golaxy so good - it played millions of games...

Author:  iopq [ Wed Jul 10, 2019 6:55 am ]
Post subject:  Re: rule discussion

Bojanic wrote:
sorin wrote:
I was under the wrong impression that bent-four is automatically dead in the Japanese rules...

I had no idea about "unremovable ko threats"; maybe never too late to learn the rules of this game :-)

I think that there is no rule that bent-four is automatically dead.
It was just explanation that you can capture it by removing all ko threats first, then starting ko.
And if there is "unremovable ko threat", then you can not remove it first.

But seriously, what are the odds of such situation happening in the game?
Bent four is rare, such seki even rarer, combination of two rare*rarer.
Answer could lead to explanation of why is Golaxy so good - it played millions of games...

Under Japanese rules, if the seki is elsewhere, it cannot be used as an unremovable ko threat because only passing is allowed in hypothetical play to remove ko restrictions, not unrelated ko threats. That's how I understand it.

Author:  YeGO [ Thu Dec 19, 2019 7:28 pm ]
Post subject:  Re: rule discussion

Under Japanese rules, its still possible for a bent four in the corner to not die. See the example on this page:
http://warp.povusers.org/go/UndeadBentFours/
Basically, two bent-4's that share a liberty can form an overall seki position, in a manner analogous to hanezeki, where initiating puts oneself at the disadvantage in a capturing race.

Related to the original topic of discussion, consider the following two board positions, assuming that we are using one of the common area-scoring rules sets and that komi is 5.5 points.

Click Here To Show Diagram Code
[go]$$B Game 1
$$ ---------------------
$$ | X . O X O . O . . |
$$ | X O O X O . O . . |
$$ | X . O X O O O . . |
$$ | O O O X X X O O O |
$$ | X X X X , X O X X |
$$ | . . X . X O O X . |
$$ | . . X . X O X X X |
$$ | . . X X X O X . O |
$$ | . . X O O O X O . |
$$ --------------------[/go]


Click Here To Show Diagram Code
[go]$$B Game 2
$$ ---------------------
$$ | X . O X O . O . . |
$$ | X O O X O . O . . |
$$ | X . O X O O O . . |
$$ | O O O X X X O O O |
$$ | X X O O X X O X X |
$$ | . X X O X O O X . |
$$ | . . X X X O X X X |
$$ | . . X . X O X . O |
$$ | . . X X X O X O . |
$$ --------------------[/go]


I had previously posted about these positions in an OGS forums post:
https://forums.online-go.com/t/life-and ... ?u=yebellz

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