Next: Another Ko Example, Previous: Ko, Up: Tactical Reading
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.