EVENT
BOARDSIZE 19
SETUP W C4 E3 O17 O16 O15 B17 C15 D14 
SETUP B Q4 P16 P17 R16 Q14 D12 D17 F16 
COM
Problem 173. Black to play.  

Black has to make an important defensive move. Where is it ?
ENDCOM
B 1 J17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White's three stones to the right are thick, so Black should
extend tightly to 1.
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  VAR
  B 1 K3
  COM
  Failure.

  If Black doesn't defend the top, but plays 1 instead, ...
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  W 2 H17
  COM
  ... White will seize the initiative by attacking with 2.
  ENDCOM
  B 3 G15
  COM
  When Black runs away with 3, ...
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  W 4 J15
  COM
  ... White makes territory at the top with 4.
  <=
  ENDCOM
  ENDVAR
W 2 L17
COM
If White answers with 2, ...
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  VAR
  W 2 O3
  MARK 1@J17
  COM
  Continuation.

  After Black 1, White will probably switch to the lower right
  and ...
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  B 3 Q6
  W 4 Q2
  B 5 R3
  W 6 L3
  COM
  ... play the joseki to 6.
  ENDCOM
  B 7 D9
  COM
  After exchanging 7 ...
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  W 8 C7
  COM
  ... for 8, ...
  ENDCOM
  B 9 O13
  COM
  ... Black will attack the three white stones by capping with
  9.
  <=
  ENDCOM
  ENDVAR
B 3 K3
COM
... Black will take the big point at 3.
<=
ENDCOM