Next: , Previous: Ko, Up: Tactical Reading


11.5 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 --decide-dragon 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.