EVENT
BOARDSIZE 19
SETUP W D17 C15 D3 C5 C6 Q5 L12 N12 O11 O12 P12 Q12 Q14 R14 R11 R10 
SETUP B D6 D8 Q3 L17 O16 Q16 R15 O13 R12 Q11 P11 O9 Q9 R9 S10 S11 
MARK \t@Q5
COM
Problem 12. Black to play.  

White has invaded inside Black's sphere of influence with the
marked stone. How should Black respond ? 
ENDCOM
B 1 Q4
MARK \t@Q5
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Bumping against the white stone with Black 1 is the strongest
move.
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  VAR
  B 1 O4
  COM
  Failure.

  The knight's move of Black 1 is the standard response to the
  white approach move, but ...
  ENDCOM
  W 2 R3
  B 3 R2
  W 4 O5
  B 5 N5
  W 6 Q2
  COM
  ... White can make sabaki with the sequence to 6.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 R4
  COM
  Failure.

  If Black 1 here, ...
  ENDCOM
  W 2 N5
  COM
  ... White will jump to 2. Either way, Black's thickness above
  has been neutralised.
  <=
  ENDCOM
  ENDVAR
W 2 R5
B 3 S4
W 4 O5
COM
The sequence to 4 can be expected. Next, ...
ENDCOM
B 5 N4
COM
... Black stakes out the territory at the bottom with 5.
ENDCOM
  VAR
  B 5 N3
  MARK @@N4
  COM
  Caution.

  Black 5 (@) in the Correct Answer is an important move. Black
  must not play 5 on the third line because ...
  ENDCOM
  W 6 P8
  B 7 P9
  W 8 M5
  MARK A@M3
  COM
  ... White can jump to 8 and her stones are savely out in the
  open. Moreover, White is threatening the attachment at A, so
  Black might have to defend against it.
  ENDCOM
  ENDVAR
W 6 P8
COM
White's stones are still insecure, so she has to escape into
the center. She peeps with 6, then ...
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B 7 P9
W 8 M6
COM
... jumps to 8.
ENDCOM
B 9 M4
COM
Black calmly defends with 8.
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W 10 M8
B 11 M9
W 12 L8
B 13 M11
W 14 M12
B 15 L9
W 16 K8
COM
With 16, White is out into the center, but ...
ENDCOM
B 17 J10
COM
... Black 17 keeps the white stones above and below separated.
<=
ENDCOM