(Referenced by 648ep)

: ( 648cx)
White connects her single stone at the upper edge.

Ko: ; ; ; ; ;

:
Black continues with the ko fight in the left centre.

; :
Please understand these two moves as replacement for White's kikashi , in the centre, which had been played too early by us.

We now know that this atari cannot be counted as an additional ko threat.
The ko fight in this subvariation is extremely close; therefore, we will take measures below to compensate for our earlier mistake.

We ask for your kind understanding that we did not adjust this and the next three diagrams.

: (A 648ex)
White captures Black's stone at the top.

Alternatively, she could have captured Black's just played stone with a move at , without affecting the final scores of the game.

:
White's peep is a ko threat that costs her one tempo in the Main Semeai (assuming that she will win the ko fight).

:
Black is forced to occupy a point that he would have to play onto anyway in the course of taking White's liberties at the left during the Main Semeai.
No Black territory has been destroyed (Capture Variation), nor has an additional White prisoner been added (Semeai Variation); therefore, the final scores of the game remained unaffected by this exchange.

Ko: ; ; ; ;

; ; ; :
These ko threats are the typical "territory-reducing" moves, which destroy one point of opponent's territory in the Capture Variation, while losing one tempo in the Main Semeai, as well as one point (i.e. an additional prisoner) in the Semeai Variation.

Ko:

: (A 648ey)

Black's throw-in (discovered late by Joachim) in the upper right loses one tempo in the Main Semeai.
However, it is none of the usual "territory-reducing" moves before, because it loses one point in both Main Variations, providing White with an additional prisoner.

In response to my guzumi (), KataGo answered with solidly connecting at (instead of our "usual" move of ). We will explain in the subvariation, why this choice was a mistake.

:
White's last ko threat is played outside the Main Semeai area, and loses one point overall, compensating for Black's previous move.
In principle, capturing at is mandatory for Black to secure independent life in the corner, after White won the ko in the centre. Here, White provided Black with an additional prisoner.
Please note that capturing with (unforced) provides Black with the same five points of territory in the lower left corner as with capturing with after White kikashi , there.

Please note that Black could have left this final exchange of ko threats unplayed; it does not affect the final scores.

; :
On the board, Black does not have any valid ko threats left.

However, we still have to compensate for our earlier mistake above, counting White's atari in the upper left centre as an additional ko threat.
Please assume this exchange as replacement for Black's kikashi , in the lower left, which has been played too early by us, too.

:
Black recaptures the ko for the last time.

:
White does not have any valid ko threats left, and so continues with the Main Semeai.

:
Black connects the ko.

Black must not play any reduction.

Capture Variation:

Compared to the superordinate variation ...

• White's cumulated gain at the upper edge is two points.
• Black destroyed one point of White's territory at the top.
• Black captured an additional White stone in the left centre, so gaining one point.
• Black got an additional point of territory there.
• Black got an additional point of territory in the lower left corner.
• White captured an additional Black stone in the upper right.
• Black destroyed an additional two points of White's territory at the top.
• White captured Black's single stone in the lower right, so gaining two points.

In total, gains and losses compensate for each other.

White + 1 => White + 1

Semeai Variation:

Compared to the superordinate variation ...

• White's cumulated gain at the upper edge is two points.
• Black destroyed one point of White's territory at the top.
• Black captured an additional White stone in the left centre, so gaining one point.
• Black got an additional point of territory there.
• Black got an additional point of territory in the lower left corner.
• White captured an additional Black stone in the upper right.
• Black captured one additional White stone at the left edge, so gaining on point.
• White captured an additional two Black stones at the top, so gaining two points.
• White captured one Black stone less in the lower right, so losing two points.

In total, White lost two points.

White + 1 => Black + 1