EVENT
BOARDSIZE 19
SETUP W F17 F3 R6 R10 N16 O17 P17 
SETUP B D16 D10 C6 D4 Q4 O3 Q17 Q16 O16 O15 Q14 
COM
Problem 30. Black to play.  

Black's position in the upper right is thick, while White's stones
at the top are thin. How should Black attack ? 
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B 1 L16
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the vital point in this position. Next ...
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  VAR
  B 1 N15
  COM
  Failure.

  Turning at 1 is not a good move ...
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  W 2 M16
  COM
  ... because White is able to make territory by running on the
  fourth line ...
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  B 3 M15
  W 4 L16
  B 5 L15
  W 6 K16
  MARK \t@F17
  COM
  ... and the marked stone works well in protecting White's left
  flank. Moreover, the two white stones on the right side neutralises
  Black's wall.
  <=
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  ENDVAR
W 2 N15
COM
Pushing up with 2 is only natural, but ...
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  VAR
  W 2 M15
  COM
  Variation.

  The diagonal move of White 2 is another possibility.
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  B 3 M18
  COM
  Again Black peeps with 3.
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  W 4 N17
  B 5 L15
  COM
  After exchanging 5 ...
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  W 6 M14
  COM
  ... for 6, ...
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  B 7 M12
  MARK \t@F17
  COM
  ... Black caps with 7. If the fight spreads to the left, the
  marked stone will be weakened.
  <=
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  ENDVAR
  VAR
  W 2 L18
  COM
  Variation.

  If White slides to 2, ...
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  B 3 K17
  COM
  ... Black will force White to run along the second line.
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  W 4 K18
  B 5 J17
  W 6 J18
  B 7 H17
  W 8 H18
  B 9 G17
  W 10 G18
  B 11 F16
  W 12 E17
  B 13 E16
  COM
  After the sequence to 13, Black's wall in the center is superior
  to White's territory at the top.
  <=
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  ENDVAR
B 3 N14
COM
... Black blocks with 3, ...
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W 4 M15
COM
... forcing White to turn with 4.
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  VAR
  W 4 L15
  COM
  Variation.

  If White attaches at 4, ...
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  B 5 M15
  COM
  ... Black will wedge in at 5.
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  W 6 M16
  B 7 M14
  W 8 L17
  B 9 K16
  W 10 K17
  B 11 J16
  W 12 J17
  B 13 H16
  W 14 H17
  B 15 G16
  COM
  The sequence to 15 is inevitable. White has secured the top,
  but Black's thickness in the center is strategically overwhelming.
  <=
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  ENDVAR
B 5 M18
COM
Next, Black peeps with 5 and ...
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W 6 N17
MARK \t@F17
COM
... White's stones are left heavy and separated from the marked
one on the left. White is in trouble. See Problem 136 for the
continuation.
<=
ENDCOM