My mistake was to play C. Now the difference between A and B is subtle. The instinctive move A leads to a fight where extra weight and loss of liberties affects the outcome of a multi-step ko, Black being komaster:
- Click Here To Show Diagram Code
[go]$$Wc Mistake 9
$$ ---------------------------------------
$$ | . X . X . . . . . . . . X . . . . O . |
$$ | X . X O X . . . . O O O X O O O O . O |
$$ | . X O O X . O . O X . X X X O X O . . |
$$ | . . O X . X O X O X X . . O X X O O . |
$$ | . . O X . X O O X O . . X O . . O X O |
$$ | . . O O X . X X X O X . X X X X O X . |
$$ | . X O . . X . . . O O O X O . O X . 0 |
$$ | . O O X X O . . O . . 2 1 O . O X . a |
$$ | . . O X O O O . O X X . X X . O . 4 . |
$$ | . . X O X O . O X , . . . . . , 3 X 8 |
$$ | . . X O . X . O X . . . . X . O . 5 6 |
$$ | . . X O . . . O X . . . . . . . . 9 7 |
$$ | . O . O X . O . . . . X . X X O O X . |
$$ | . . O . O O . X . . . . O O O X X X . |
$$ | . O X X . . X . X X X . . . . O O X . |
$$ | . . O , X X . O X O X X O O . O X . . |
$$ | . . . O O X . O X O X O O X O O X . . |
$$ | . . . . . . . O O O O . O X O X X . . |
$$ | . . . . . . . . . . . . . X X . . . . |
$$ ---------------------------------------[/go]
- Click Here To Show Diagram Code
[go]$$Wc Mistake 9
$$ ---------------------------------------
$$ | . X . X . . . . . . . . X . . . . O . |
$$ | X . X O X . . . . O O O X O O O O . O |
$$ | . X O O X . O . O X . X X X O X O . . |
$$ | . . O X . X O X O X X . . O X X O O 3 |
$$ | . . O X . X O O X O . . X O . . O B O |
$$ | . . O O X . X X X O X . X X X X O # 5 |
$$ | . X O . . X . . . O O O X O . O X 7 X |
$$ | . O O X X O . . O . . X O O . O X 2 1 |
$$ | . . O X O O O . O X X . X X . O . X 4 |
$$ | . . X O X O . O X , . . . . . , O X X |
$$ | . . X O . X . O X . . . . X . O a O X |
$$ | . . X O . . . O X . . . . . . 6 0 O O |
$$ | . O . O X . O . . . . X . X X O O X . |
$$ | . . O . O O . X . . . . O O O X X X . |
$$ | . O X X . . X . X X X . . . . O O X . |
$$ | . . O , X X . O X O X X O O . O X . . |
$$ | . . . O O X . O X O X O O X O O X . . |
$$ | . . . . . . . O O O O . O X O X X . . |
$$ | . . . . . . . . . . . . . X X . . . . |
$$ ---------------------------------------[/go]
at the squared stone,
at the circled. Next is A.
Without the exchange in the first diagram, White can win the ko and allow Black an easy way out with the top group. With the exchange, Black has more damaging forcing moves. Or so it seems - I haven't checked the full gamut of variations here.
Obviously I didn't intend to have this superhuman analysis to be part of the exercise.