EVENT
BOARDSIZE 19
SETUP W C16 D17 E17 B14 L16 M16 O17 O15 R8 R6 S4 C6 
SETUP B D14 E16 F16 F17 K17 L17 M17 Q16 Q14 R10 R3 Q4 O4 D4 
MARK \t@L16 \t@M16 \t@O17 \t@O15
COM
Problem 46. Black to play.  

How should Black attack the four marked white stones ? 
ENDCOM
B 1 M14
MARK A@N16
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 blocks White's escape route into the center and aims
at A.
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  VAR
  B 1 Q18
  COM
  Failure.

  Defending the corner with 1 is passive.
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  W 2 L13
  COM
  White can gingerly jump out into the center with 2.
  <=
  ENDCOM
  ENDVAR
W 2 N16
COM
If White defends there, ...
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  VAR
  W 2 N14
  COM
  If White attaches with 2, ...
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  B 3 N16
  COM
  ... Black hanes at the vital point with 3.
  ENDCOM
  W 4 N17
  B 5 M15
  W 6 N15
  MARK 3@N16
  COM
  White captures the stone at 3 with 6, but ...
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  B 7 K15
  COM
  ... Black casts a net with 7, making a thick position facing
  the left. White's stones are still without eye shape.
  <=
  ENDCOM
  ENDVAR
B 3 O13
COM
... Black continues the attack with 3.
<=
ENDCOM