: ( 334)
Here we show a simple method to evaluate, whether playing the ko will be a suitable option for Black.

Black lets White connect the ko, and thereafter plays a "normal" semeai.

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Eventually, Black is one move behind. He cannot give atari at , and capturing two White stones with a move at will not help either.

Assuming the sequence shown, if there still were to be an open ko on the right side, White would not have played (= ), and Black would not have played as well.

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We took , and , off the board, for demonstrative purposes.

Black re-captures the ko now.

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The assumption that both sides had the same number of valid external ko-threats is equivalent to the assumption that both sides had run out of valid external ko-threats. We use the latter one to simplify the presentation here.

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Capturing White's two stones inside his larger eye wins the game for Black now.

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Black gives atari on White's centre group, and will take it off the board with his next move.

So ko is really a valid option for Black here.