EVENT
BOARDSIZE 19
SETUP W C18 D18 D17 E19 F18 G17 K17 R14 R12 R11 Q10 Q8 R6 S4 K4 L4 K7 C3 D4 D6 D8 C10 C13 
SETUP B M17 Q16 R16 Q14 Q12 R3 Q4 O4 K3 J3 H5 F3 D2 E10 E13 C17 C16 D16 E18 E17 F16 D15 E15 
MARK \t@C18 \t@D18 \t@D17 \t@E19 \t@F18 \t@G17 \t@K17
COM
Problem 86. Black to play.  

How should Black attack the marked white stones ? 
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B 1 J18
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

The placement of Black 1 is on the vital point.
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W 2 J17
COM
Blocking from above with White 2 is best.
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  VAR
  W 2 K18
  MARK 1@J18
  COM
  Variation.

  In answer to Black 1, blocking at 2 seems to be a viable option,
  but ...
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  B 3 J17
  W 4 J16
  B 5 K16
  W 6 H17
  B 7 J15
  W 8 H16
  B 9 L16
  COM
  ... Black builds up a large moyo in the center with the sequence
  to 9. Black ends in sente ...
  ENDCOM
  W 10 B17
  COM
  ... since White must make eyes in the corner with 10.
  <=
  ENDCOM
    VAR
    W 10 PASS
    COM
    ... Black builds up a large moyo in the center with the sequence
    to 9. Black ends in sente ...
    ENDCOM
    B 11 H18
    MARK @@B17
    COM
    What happens if White neglects to play at @ ?

    Black will turn at 11.
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    W 12 G18
    B 13 L18
    COM
    After exchanging 13 ...
    ENDCOM
    W 14 H19
    COM
    ... for 14, ...
    ENDCOM
    B 15 F19
    COM
    ... Black throws in a stone at 15. Making two eyes in now problematic
    for White.
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    W 16 G19
    COM
    If White captures at 16, ...
    ENDCOM
    B 17 B18
    COM
    ... Black blocks with 17.
    ENDCOM
    W 18 H15
    B 19 J14
    W 20 H14
    B 21 H13
    W 22 G13
    COM
    White tries to get out into the center with the sequence to 22,
    but ...
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    B 23 H12
    COM
    ... White is trapped after Black 23.
    <=
    ENDCOM
    ENDVAR
  ENDVAR
B 3 K18
W 4 H18
COM
After White 4, ...
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B 5 L17
MARK A@K16 B@B18
COM
... Black is threatening to peep at A and hane at B, so White's
stones are still insecure.
<=
ENDCOM