(Referenced by 333; 957)

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: ( 944)
Black's atari at is played too early. White simply connects the ko with . Please note that there is no White stone at yet, so is no atari.

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Here, White is able to capture at , before occupying Black's last outside liberty with .

Black is unable to give atari at . White wins the semeai outright.

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After - in the sequence shown above - if there was a White stone at already; lets say mistakenly played instead of one of the -stones (one of which we will remove now for further presentation), ...

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... White would be forced to connect at . Thereafter, if she played at , she would have lost a move, compared to the sequence shown above, and Black could capture her centre group.

Ko: ;

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This implies that - after White's connection at - the ko-shape would have been transferred to the upper edge, and the ko-fight would continue with White capturing the ko with there.

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Again, Black needed one external ko-threat for success.
White could have simply answered Black's atari at (with his move ) at as well.

Black's atari on the upper edge is a valid ko-threat, only if White's centre group has less than four liberties, or White has played already.