$$Bc Mistake 11
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X X . . . . . . . . . . . . |
$$ | . . X X O O O X . . . . . O . . . . . |
$$ | . . O O b . . . . , . . . . . O . . . |
$$ | . . . . . c . . . . . . . X . . O . . |
$$ | . . O . . . . . . . . . . . . X X . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . . X . . . O . X O . O X . . . . . |
$$ | . O O X . X O . . X . O X X . X X . . |
$$ | . . X , X X X O . , . O O . . X O . . |
$$ | . a X X O . O X X X . . . O . O O . . |
$$ | . . O O . O O O O X . . X O . O . . . |
$$ | . . . . O . X X . X . X X X X X O . . |
$$ | . . O O . . O X . X . . O . . . O . . |
$$ | . . X . . X O O O O . X O . . . . . . |
$$ | . . . X . . . . . X . . O X X X X X . |
$$ | . . . . . X . O . . . . O X O O O X . |
$$ | . . . . . . . . . . . O . O . . . O . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------
[go]$$Bc Mistake 11
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X X . . . . . . . . . . . . |
$$ | . . X X O O O X . . . . . O . . . . . |
$$ | . . O O b . . . . , . . . . . O . . . |
$$ | . . . . . c . . . . . . . X . . O . . |
$$ | . . O . . . . . . . . . . . . X X . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . . X . . . O . X O . O X . . . . . |
$$ | . O O X . X O . . X . O X X . X X . . |
$$ | . . X , X X X O . , . O O . . X O . . |
$$ | . a X X O . O X X X . . . O . O O . . |
$$ | . . O O . O O O O X . . X O . O . . . |
$$ | . . . . O . X X . X . X X X X X O . . |
$$ | . . O O . . O X . X . . O . . . O . . |
$$ | . . X . . X O O O O . X O . . . . . . |
$$ | . . . X . . . . . X . . O X X X X X . |
$$ | . . . . . X . O . . . . O X O O O X . |
$$ | . . . . . . . . . . . O . O . . . O . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
Things are hot in the upper left. Should Black keep White disconnected at ''a'', cut to capture at ''b'' or play the vital point of ''c''?
Black's choice lost 2,5 points but shifted the balance to White again and was the start of lots of trouble.
Black is not alive, D10 is a false eye, and if White can somehow manage to pull out the marked stones, then White separates two weak black groups, which may cause trouble.
If Black b, then White may think about sacrificing F17.
The situation is hard to read but "c" seems a good choice, which defends D13 and at the same time aims at "b" or at swallowing the marked white stones.
So I vote for "c".
$$Bc Mistake 11
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X X . . . . . . . . . . . . |
$$ | . . X X O O O X . . . . . O . . . . . |
$$ | . . O O b . . . . , . . . . . O . . . |
$$ | . . . . . c . . . . . . . X . . O . . |
$$ | . . O . . . . . . . . . . . . X X . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . . X . . . W . X O . O X . . . . . |
$$ | . O O X . X W . . X . O X X . X X . . |
$$ | . . X , X X X W . , . O O . . X O . . |
$$ | . a X X O . O X X X . . . O . O O . . |
$$ | . . O O . O O O O X . . X O . O . . . |
$$ | . . . . O . X X . X . X X X X X O . . |
$$ | . . O O . . O X . X . . O . . . O . . |
$$ | . . X . . X O O O O . X O . . . . . . |
$$ | . . . X . . . . . X . . O X X X X X . |
$$ | . . . . . X . O . . . . O X O O O X . |
$$ | . . . . . . . . . . . O . O . . . O . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------
[go]$$Bc Mistake 11
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X X . . . . . . . . . . . . |
$$ | . . X X O O O X . . . . . O . . . . . |
$$ | . . O O b . . . . , . . . . . O . . . |
$$ | . . . . . c . . . . . . . X . . O . . |
$$ | . . O . . . . . . . . . . . . X X . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . . X . . . W . X O . O X . . . . . |
$$ | . O O X . X W . . X . O X X . X X . . |
$$ | . . X , X X X W . , . O O . . X O . . |
$$ | . a X X O . O X X X . . . O . O O . . |
$$ | . . O O . O O O O X . . X O . O . . . |
$$ | . . . . O . X X . X . X X X X X O . . |
$$ | . . O O . . O X . X . . O . . . O . . |
$$ | . . X . . X O O O O . X O . . . . . . |
$$ | . . . X . . . . . X . . O X X X X X . |
$$ | . . . . . X . O . . . . O X O O O X . |
$$ | . . . . . . . . . . . O . O . . . O . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
$$Bc Mistake 11
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X X . . . . . . . . . . . . |
$$ | . . X X O O O X . . . . . O . . . . . |
$$ | . . O O b . . . . , . . . . . O . . . |
$$ | . . . . . c . . . . . . . X . . O . . |
$$ | . . O . . . . . . . . . . . . X X . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . . X . . . O . X O . O X . . . . . |
$$ | . O O X . X O . . X . O X X . X X . . |
$$ | . . X , X X X O . , . O O . . X O . . |
$$ | . a X X O . O X X X . . . O . O O . . |
$$ | . . O O . O O O O X . . X O . O . . . |
$$ | . . . . O . X X . X . X X X X X O . . |
$$ | . . O O . . O X . X . . O . . . O . . |
$$ | . . X . . X O O O O . X O . . . . . . |
$$ | . . . X . . . . . X . . O X X X X X . |
$$ | . . . . . X . O . . . . O X O O O X . |
$$ | . . . . . . . . . . . O . O . . . O . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------
[go]$$Bc Mistake 11
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X X . . . . . . . . . . . . |
$$ | . . X X O O O X . . . . . O . . . . . |
$$ | . . O O b . . . . , . . . . . O . . . |
$$ | . . . . . c . . . . . . . X . . O . . |
$$ | . . O . . . . . . . . . . . . X X . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . . X . . . O . X O . O X . . . . . |
$$ | . O O X . X O . . X . O X X . X X . . |
$$ | . . X , X X X O . , . O O . . X O . . |
$$ | . a X X O . O X X X . . . O . O O . . |
$$ | . . O O . O O O O X . . X O . O . . . |
$$ | . . . . O . X X . X . X X X X X O . . |
$$ | . . O O . . O X . X . . O . . . O . . |
$$ | . . X . . X O O O O . X O . . . . . . |
$$ | . . . X . . . . . X . . O X X X X X . |
$$ | . . . . . X . O . . . . O X O O O X . |
$$ | . . . . . . . . . . . O . O . . . O . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
Surely this must be 'c'? Definitely not 'b'...white will give up the stones. 'a' at least looks at the white stones below, but Black's corner isn't that strong and I'm afraid of losing sente.
I'd play 'c' with an eye on attacking the white three stones around H10. Later, I'd want to play B5 to secure the corner while aiming at F6.
'b' looks dangerous, as White might sacrifice either the two or the three stones and then seriously attack the black group.
'a' look dangerous, as the white groups it disconnects do not seem very weak (F6/F9 is countered by E4), and since 'a' does nothing else (no eyes, no connection), White can then attack the black group.
A good system naturally covers all corner cases without further effort.