EVENT
BOARDSIZE 19
SETUP W D10 D4 K16 K17 O17 O16 P16 Q4 R5 P6 Q6 R7 
SETUP B C16 D12 H16 P17 Q17 R16 R14 Q7 R6 R3 Q3 O3 
MARK C@C10 A@E16 B@F12
COM
Problem 134. Black to play.  

The joseki in the lower right corner has ended with sente for
Black. He must now turn his attention to his thin position at
the top. Where should he play ? 
ENDCOM
B 1 E16
MARK C@C6 B@C8 A@E17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Even though Black's position at the top is open  from the side,
he must make a corner enclosure with 1 and not at A. Next, he
can aim at B or C, ...
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  VAR
  B 1 C10
  COM
  Failure.

  Black 1...
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  W 2 C9
  B 3 C11
  COM
  ... and 3 are not good because they strengthen White's position
  on the left.
  ENDCOM
  W 4 D9
  COM
  This is a big loss for Black.
  <=
  ENDCOM
  ENDVAR
W 2 D6
COM
... so White defends with 2.
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B 3 L3
COM
In response to Black 3, ...
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W 4 J4
COM
... White 4 is a good move.
<=
ENDCOM