First to answer the original question, just block is correct. That exchange is white's right, but doesn't really help the corner live so it will likely still be the same ko as if white didn't make that exchange, and that exchange is a slight loss for white (it makes black stronger on the outside and increase his influence on the left side).
- Click Here To Show Diagram Code
[go]$$Wc "trick play"
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . 2 X . . . . . . .
$$ | . 1 X . . . . . . .
$$ | . . O X . . . . . ,
$$ | . . O X . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . . .
$$ ---------------------[/go]
The continuation splittingfield worries about is indeed more likely for white, but it's locally dead with the bad aji of the cut, the same as without the bad exchange. White can then try
a or
b. The cut of
a doesn't quite capture anything yet, whilst
b clamp might escape with a 2nd line crawl that's usually worse than death.
- Click Here To Show Diagram Code
[go]$$c "trick play"
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . 1 X . . . . . . .
$$ | 7 O X . . . . . . .
$$ | . 5 O X . . . . . ,
$$ | . 6 O X a . . . . .
$$ | . . 4 2 3 b . . .
$$ | . . . . . . . . . .
$$ ---------------------[/go]
Note that in these variations white's hane-block exchange is a bad exchange as they made black stronger and with more eyeshape than without:
- Click Here To Show Diagram Code
[go]$$Wc normal
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . . X . . . . . . .
$$ | . 6 X . . . . . . .
$$ | . 4 O X . . . . . ,
$$ | . 5 O X . . . . . .
$$ | . . 3 1 2 7 . . .
$$ | . . . . . . . . . .
$$ ---------------------[/go]
And now to correct lodestone's comments:
lodestone wrote:
It seems like black's C5 move might better be at B5.
Probably not unless the outside is special. White can live small like this:
- Click Here To Show Diagram Code
[go]$$Wc
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . . X . . . . . . .
$$ | . B 2 . . . . . . .
$$ | . 1 O X . . . . . ,
$$ | 5 . O X . . . . . .
$$ | . 3 . 4 . . . . . .
$$ | . . . . . . . . . .
$$ ---------------------[/go]
Or start the ko and black's outside is super weak (at
a) if white wins it
- Click Here To Show Diagram Code
[go]$$Wc
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . . X . . . . . . .
$$ | . B . a . . . . . .
$$ | . . O X . . . . . ,
$$ | . . O X . . . . . .
$$ | . 3 . 1 2 . . . . .
$$ | . . . . . . . . . .
$$ ---------------------[/go]
Or play somewhat heavier and just try living:
- Click Here To Show Diagram Code
[go]$$Wc
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . . X . . . . . . .
$$ | . B . 6 . . . . . .
$$ | . 5 O X . . . . . ,
$$ | . . O X . 4 . . . .
$$ | 7 . 3 1 2 . . . . .
$$ | . . . . . . . . . .
$$ ---------------------[/go]
or if black extends then clamp either gets
a in sente to live or escapes on 2nd line:
- Click Here To Show Diagram Code
[go]$$Wc
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . . X . . . . . . .
$$ | . B . 4 . . . . . .
$$ | . . O X . . . . . ,
$$ | . . O X . . . . . .
$$ | . . 3 1 2 5 . . . .
$$ | . . . a . . . . . .
$$ ---------------------[/go]
lodestone wrote:
But in your continuation, doesn't B6 A4 B2 kill?
Maybe, but with very bad aji. The much better way to kill is:
- Click Here To Show Diagram Code
[go]$$c "trick play"
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . X X . . . . . . .
$$ | . O X . . . . . . .
$$ | O 4 O X . . . . . ,
$$ | 6 3 O X . . . . . .
$$ | . 2 1 . 7 . . . . .
$$ | . 5 . . . . . . . .
$$ ---------------------[/go]
lodestone wrote:
Or if W doesn't make a tiger's mouth, there's still a ko.
No, dead:
- Click Here To Show Diagram Code
[go]$$c "trick play"
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . X X . . . . . . .
$$ | 7 O X . . . . . . .
$$ | 8 O O X . . . . . ,
$$ | 3 . O X . . . . . .
$$ | 9 2 4 1 . . . . . .
$$ | . 6 5 . . . . . . .
$$ ---------------------[/go]