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Tsumego From Games 20
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    Keywords: Problem

This came out today in a game. White thought that the group at the corner is alive. Is this so? --unkx80

[Diagram]
Diag.: Status? (All outer stones are alive.)


[Diagram]
Diag.: What happened during the game.

During the game, Black played 1. After Black 5, a and b are miai, and White is killed by shortage of liberties. White resigned soon after that.


[Diagram]
Diag.: Killed also.

White 2 provides another path of resistance. However, up to the Black 7, the White group is dead by bulky five. It does not matter if White 2 plays at 4 first.

But--


[Diagram]
Diag.: The correct play by White.

The sagari by White at 2 is the correct move which both players failed to see. To stop White from connecting underneath Black may play at 3, but then White 4 ensures life. After that, a and b are miai.


[Diagram]
Diag.: The actual solution.

The real solution is actually the placement at 1. The best sequence is from 2 to 7, which results in a two stage ko.

Dieter: the 4-5 exchange makes it a rectangular six in the corner with one outside liberty. In that case, the 1-2 point makes it ko. Just this note to link to other Go knowledge on SL.


[Diagram]
Diag.: Not as good.

White 2 followed by 4 is yet another variation. Up to Black 7, it is still a ko, but the ko is worse off for White compared to the previous diagram.



Bill Spight: A wonderful position, unkx80! :-)

The ko is certainly a two-stage ko, in the sense that it has two stages. Because of the throw-in it is unusual, but this case has another quirk, thanks to the local ko threat to connect. I'll discuss this further at Rogue Ko.

unkx80: Thanks. But shamefully, I was Black. =)



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