( 988)

(Referenced by 352)

Black's -stone must be moved one point to the left, to , in order to enable a ko-shape in this area.

White's -stone creates the external ko-shape on the upper edge. This additional stone is compensated for by the additional -stone for Black. Without this stone, Black could not wholly approach White's centre group from the left.

The liberty count on the right is still five liberties for White (w, m, 3 * c) against seven liberties for Black (3 * c, 3 * b, k), so Black needs one external ko-threat to win. Black's -stone reduced the liberties of White's upper partial group on the left side, but - even after some preparatory moves in the nakade - Black will not gain the initial external ko-threat there, which would be mandatory to win the ko-fight. This is because Black's large group in the upper right is below the Punishment Semeai Liberty Level, so there are no valid ko-threats on the left side of the board in the moment.

White could have even improved her position in the top left corner beforehand by choosing the sente combination of , , to prepare the ground for forthcoming ko-threats there. Please note that White's upper string on the left side did not lose an additional liberty by this measure. We also added the beginning of the Nakade Sequence on the left side ( / ).

Black's protective move is at now. A protective move at , instead, would enable an additional White ko-threat.

!!! The precondition concerning this protective move must be kept in mind! With positions in the upper left corner, where this protective move is not mandatory, one of White's stones on the right side must be taken off the board, resulting in one more liberty for Black's group in the upper right, favouring Black. !!!

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Black uses his atari in the hanezeki to avoid connecting the hanezeki's tail, while White's centre group has still five liberties.

Ko:

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Black needs one external ko-threat to win.

is not played already, so Black's atari at on the upper edge is no valid ko-threat at the moment.
A move at on the left side is no valid ko-threats either, because White's centre group has four liberties left ( 951).

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This liberty occupying move, however, changes the dependencies with the left side of the board.

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Black's atari with the nakade on the left side is a valid ko-threat now, because White's centre group has been reduced to three liberties only.

But please remember the precondition that Black must have been able to play the additional external ko-threat (at ) before, which he does not really own in the problem.

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However, White will need her own ko-threats in the top left corner now. This means that the external ko on the upper edge cannot be called "technically correct" in conjunction with Black's larger eye in the centre.

Ko: ;

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White could live with her groups in the hanezeki, with a move at , after having used the moves , , and , as ko-threats. However, this will not be enough to win the game, because her centre group will be taken off the board.

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Black wins the semeai by one move.

This is the same result as with Black's larger eye alone, thus the additional external ko-shape on the upper edge does not have an effect on the outcome here.

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With her fourth move in a row, White kills Black's -group on the left side that looked so rock solid before. But her -groups in the hanezeki are dead, so Black will win the game with ease.

!!! White could have pre-empted the death of her groups in the hanezeki, if she had inserted the gote-sente-combination of , , , instead of simply occupying Black's hanezeki-directed liberty with a move at . Black's move at , used as a ko-threat, will become suicidal then. !!!

In this case, capturing only White's centre group is not enough for Black. White will win the game by the surprising sequence in the lower left corner, as will be shown next.