Let's enter the world of amateurs ...

: ( 2039b)
Dany's Atekomi in the left centre will trigger a sudden ko-fight and finally benefit White by one point overall.

White's last move created an additional approach-move liberty for her left-hand group, and so does not lose a tempo in the Main Semeai.

Four Black moves are now needed for filling White's liberties in the left centre (3 x ; 1 x ), instead of only three before (3 x ; in principle).
But we have to take into account the White did not occupy one of Black's Main Semeai Liberties in the upper right, so the net effect in Main Semeai Liberties is zero.

Additionally, Black must find a defense against White's cut at .

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Black gives atari at White's just played stone.

Black will lose the ensuing ko-fight anyway, so he might consider simplifying the issue by immediately connecting at .
In principle, this immediate connection would lose one tempo in the Main Semeai, but which Black could not use to reduce White's territory in the Capture Variation anyway, as we will see below.

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White, however, must create and fight a ko, at least for a while.

If she simply connected with , instead, Black would take the opportunity to capture White's single stone at the upper edge with , gaining two points at the cost of one Main Semeai Liberty, an effective advantage of one point.

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This capture starts Dany's Ko.

For further details of our analysis at the time, please refer to the history of our investigations ...
648mk

As already mentioned above, we intensively investigated this line in a "no-kikashi" approach, i.e. without all of Black's kikashi , played, hoping that these could be utilised as Black ko-threats ...
648cx

During the course of this investigation, KataGo found a surprising number of White ko-threats in Black's upper left corner, including and .
In the very end, KataGo's 2021 version provided White with a total of four ko-threats in this small area ...
648qa

On the other hand, Joachim discovered that Black's moves at and in the bamboo-joint ( had been exchanged for much earlier) were no valid ko-threats for him, as White was able to successfully ignore these ...
648qa

In order to profit from the ko-fight in the left centre, Black must connect the ko at .
In this case, will become an eye, rendering White's forthcoming tesuji , , ineffective.

Black's last move did not lose a tempo in the Main Semeai.
He still needed four moves for filling White's liberties in the left centre (3 x ; 1 x ), the same as above. However, he would save the connection at , so the net effect would be zero.

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White uses the atari at the upper edge as a ko-threat.

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White recaptures the ko.

White's last move did not lose a Main Semeai Liberty.

In principle, lost one tempo in the Main Semeai, but Black has to fill five points (3 x ; 2 x ) in the left centre now, instead of only four points before, so the net effect is zero.

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Black defends against a threatening damezumari.

If he played elsewhere, instead, White would give atari at his two stones at the left edge with . After , would be another atari that Black could not connect with ; White would simply capture with .
Black's only remaining option would be to recapture the ko with . However, this ko-fight would be too big to win, as Black could not compensate for White's ko-threats after the capture of the nakade.

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White's push in the lower left corner is tesuji, effectively reducing Black's territory.

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Black is forced to connect here, in order to secure an independently alive formation in the lower left corner alone.

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White secures profit outside the Main Semeai Area by connecting her single stone at theupper edge.

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Black is unable to win the ko-fight, so we will simplify the issue here.

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White must connect the ko - sooner or later.

Otherwise, Black would be able to utilise his liberty-occupying moves in the Main Semeai as ko-threats at an opportune moment.
But White cannot afford to let Black capture the ko and connect. Black would gain two points unnecessarily, as already explained above.

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The Main Semeai continues.

...

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Black plays 3/4 reductions.

If he fully utilised his surplus Main Semeai Liberties for 4/4 reductions, the final scores of the Main Variations would become White + 1 / White + 3.

Black will have to connect at and in the centre in the course of the ensuing Main Semeai.

After White's decision point, Black will have to capture at , as White could give atari at his large group in the left centre.

Capture Variation:

Compared to the superordinate variation, ...

• Black destroyed one point of White's territory in the upper right corner.
• White's cumulated gain in the lower left corner is two points.

In total, White gained one point.

White + 1 => White + 2

Semeai Variation:

Compared to the superordinate variation, ...

• White captured one Black stone more in the upper right corner, so gaining one point.
• Black captured one White stone more in the lower left corner, so White's net gain there is zero points.

In total, White gained one point.

White + 1 => White + 2

2039b