EVENT
BOARDSIZE 19
SETUP W L17 S17 R16 R15 Q14 Q13 R12 Q4 R5 O3 B4 C6 C9 
SETUP B D17 C15 J17 R18 R17 P17 Q16 Q15 P14 K3 D4 C3 F3 
MARK \t@L17
COM
Problem 16. Black to play.  

How can Black use his wall on the right side to attack the marked
white stone ? 
ENDCOM
B 1 K15
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

The knight's move of Black 1 is the most effective way to attack.
The white stone at the top is now in a pinch.
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  VAR
  B 1 J15
  COM
  Failure.

  Jumping out into the center with Black looks like a normal move,
but it is slack.
  ENDCOM
  W 2 L15
  COM
  White jumps to 2, and Black doesn't have a good follow-up, so
  White's stones have escaped.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 K16
  COM
  Failure.

  The diagonal move of Black 1 is also slack.
  ENDCOM
  W 2 M15
  B 3 K14
  W 4 M13
  MARK 2@M15
  COM
  After 2 and 4, White's stones are out in the center.
  ENDCOM
  ENDVAR
  VAR
  B 1 L15
  COM
  Failure.

  Capping is another common attacking technique, but, after Black
  caps with 1, ...
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  W 2 K16
  COM
  ... White escapes ...
  ENDCOM
  B 3 J14
  W 4 K15
  B 5 K14
  W 6 H16
  B 7 H17
  W 8 G16
  B 9 G17
  W 10 H14
  B 11 H13
  W 12 G14
  MARK A@L14
  COM
  ... with the sequence to 12. Black has failed to make any significant
  gains, and White has the cut at A to exploit.
  <=
  ENDCOM
  ENDVAR
W 2 L15
COM
Trying to break out into the center with White 2 seems reasonable,
but ...
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  VAR
  W 2 M15
  COM
  Variation.

  Against the knight's move of White 2,
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  B 3 M16
  COM
  ... Black attaches at 3. He then ...
  ENDCOM
  W 4 L16
  B 5 L15
  W 6 N16
  B 7 M14
  W 8 M17
  B 9 N14
  MARK A@D10
  COM
  ... sacrifices this stone and confines White to the top with the
  sequence to 9. The white stones are alive, but just like in the
  Correct Answer, Black gets a thick wall and he can then create
  a moyo by attacking the white stone on the left with a shoulder
  hit at A.
  <=
  ENDCOM
  ENDVAR
  VAR
  W 2 K16
  COM
  Variation.

  The diagonal attachment of White 2 is not good. After ...
  ENDCOM
  B 3 J16
  W 4 L15
  B 5 L14
  W 6 K14
  B 7 J15
  W 8 M14
  B 9 L13
  W 10 M15
  B 11 M13
  COM
  ... Black 11, White can't live because there are no defects in
  Black's position to exploit.
  <=
  ENDCOM
  ENDVAR
B 3 L14
W 4 M15
B 5 M14
W 6 O15
B 7 P15
W 8 N14
B 9 N13
W 10 J18
B 11 H18
W 12 K18
B 13 H17
COM
... Black can confine White to the top with the sequence to 13.
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W 14 N17
B 15 O18
W 16 N18
MARK 14@N17
COM
White barely lives in gote with 14 and 16, but ...
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B 17 D10
COM
... Black can now build a moyo in the center with the shoulder
hit of 16.
<=
ENDCOM