Seki Example 4

muchen1999 [4d] vs. TheCaptain [4d] 2009-03-25  

This was a game between TheCaptain (black) and muchen1999. As usual with TheCaptain's games, a complicated series of capturing races and ko fights ensued, and in this case, led to the marked sekis.

in response to was a mistake. It was not necessary to maintain the seki and cost black one point in the form of a prisoner (Japanese rules). Black lost the game by just half a point.

http://files.gokgs.com/games/2009/3/26/muchen1999-TheCaptain.sgf

For those of you who are wondering why was a mistake, here's why.

White can't start a ko ( = tenuki, captures)  

There is no ko in the lower right corner because Black can always respond to with (which is a double atari).