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14.5 Another Ko Example

We now consider an example to show why the komaster is reset to EMPTY if the ko is resolved in the komaster's favor. This means that the ko is filled, or else that is becomes no longer a ko and it is illegal for the komaster's opponent to play there.

The position resulting under consideration is in the file `regressions/games/ko5.sgf'. This is the position:

 
 . . . . . . O O 8
 X X X . . . O . 7
 X . X X . . O . 6
 . X . X X X O O 5
 X X . X . X O X 4
 . O X O O O X . 3
 O O X O . O X X 2
 . O . X O X X . 1
 F G H J K L M N

We recommend studying this example by examining the variation file produced by the command:

 
gnugo -l ko5.sgf --quiet --decidestring L1 -o vars.sgf

The correct resolution is that H1 attacks L1 unconditionally while K2 defends it with ko (code KO_A).

After Black (X) takes the ko at K3, white can do nothing but retake the ko conditionally, becoming komaster. B cannot do much, but in one variation he plays at K4 and W takes at H1. The following position results:

 
 . . . . . . O O 8
 X X X . . . O . 7
 X . X X . . O . 6
 . X . X X X O O 5
 X X . X X X O X 4
 . O X O O O X . 3
 O O X O . O X X 2
 . O O . O X X . 1
 F G H J K L M N

Now it is important the `O' is no longer komaster. Were `O' still komaster, he could capture the ko at N3 and there would be no way to finish off B.


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