Rogue Ko

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Rogue kos are mentioned in Mathematical Go.[1] For the most part they seem to act like approach kos with additional options.

This example comes from Tsumego from games 20. Black will usually prefer to make a bent four shape in the corner rather than play the approach move outside, because it allows him to take the second ko first. Here White's local ko threat makes the approach move better, as a rule.

[Diagram]

Throw in ko

To prevent Black from making Bent Four, White 1 throws in to make a ko. White can use White 3 to set up a local ko threat for the direct ko.

[Diagram]

Sacrifice

This is the usual continuation for this kind of throw-in ko in the corner. Black makes the bent four shape and White takes.

[Diagram]

Sacrifice (continued) 8 at 4

Then Black plays on the 2-1 (B3) and White throws in (W4) to make the direct ko. Now White can use the local threat at W6 to effectively take the ko first. (Note that Black may now have ko threats, starting at a.)

[Diagram]

Approach move

Because of White's local threat, Black's sacrifice does not give him the advantage of taking ko first in the second stage. It is better, looking only in the corner, to eliminate White's ko threat with Black 1, which is also an approach move. This saves 1 point locally, no matter who wins the ko.

Black should play the sacrifice only when he makes a gain worth more than 1 point through the aji at a.

(What a wonderful game! :-))

--Bill Spight


[1]But are not defined (!)


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