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14.4 A Ko Example

To see the komaster scheme in action, consider this position from the file `regressions/games/life_and_death/tripod9.sgf'. We recommend studying this example by examining the variation file produced by the command:

 
  gnugo -l tripod9.sgf --decidedragon C3 -o vars.sgf

In the lower left hand corner, there are kos at A2 and B4. Black is unconditionally dead because if W wins either ko there is nothing B can do.

 
 8 . . . . . . . .
 7 . . O . . . . .
 6 . . O . . . . .
 5 O O O . . . . .
 4 O . O O . . . .
 3 X O X O O O O .
 2 . X X X O . . .
 1 X O . . . . . .
   A B C D E F G H

This is how the komaster scheme sees this. B (i.e. X) starts by taking the ko at B4. W replies by taking the ko at A1. The board looks like this:

 
 8 . . . . . . . .
 7 . . O . . . . .
 6 . . O . . . . .
 5 O O O . . . . .
 4 O X O O . . . .
 3 X . X O O O O .
 2 O X X X O . . .
 1 . O . . . . . .
   A B C D E F G H

Now any move except the ko recapture (currently illegal) at A1 loses for B, so B retakes the ko and becomes komaster. The board looks like this:

 
 8 . . . . . . . .         komaster: BLACK
 7 . . O . . . . .         kom_pos: A2
 6 . . O . . . . .
 5 O O O . . . . .
 4 O X O O . . . .
 3 X . X O O O O .
 2 . X X X O . . .
 1 X O . . . . . .
   A B C D E F G H

W takes the ko at B3 after which the komaster is GRAY and ko recaptures are not allowed.

 
 8 . . . . . . . .         komaster: GRAY
 7 . . O . . . . .         kom_pos: B4
 6 . . O . . . . .
 5 O O O . . . . .
 4 O . O O . . . .
 3 X O X O O O O .
 2 . X X X O . . .
 1 X O . . . . . .
   A B C D E F G H

Since B is not allowed any ko recaptures, there is nothing he can do and he is found dead. Thus the komaster scheme produces the correct result.


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