Main Solution  

is the key move that aims at the cut at a and effectively stops from escaping.

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White's best try would be .

After , Black has to fight a ko for life.

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Black starts the ko with and . By playing at , black takes the ko first.

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Black has taken the ko with , so white has to find the first ko threat. Furthermore, black has two internal ko threats at a and b.

Solution 2

Another Solution  

Black can switch the order of play by starting with .

However, Black still can't avoid the ko.

 

here doens't stop from escaping; White can kill without ko.

 

is the same for at x.

 

Starting with or x is a good way to kill one's own group in gote.