is necessary to prevent White from forming a ko. After , is the vital point: White can't play 5 due to shortage of liberties, and it becomes a false eye. If white plays any other move after , black plays at .
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is necessary to prevent White from forming a ko. After 
, 
is the vital point: White can't play 5 due to shortage of liberties, and it becomes a false eye. If white plays any other move after