EVENT
BOARDSIZE 19
SETUP W C17 D16 D14 D4 O17 Q18 R18 R17 R16 R14 Q14 Q12 P13 O13 N14 M14 
SETUP B Q4 D18 F17 F15 M17 O16 P18 P17 Q17 Q16 Q15 P14 O14 R12 Q11 N13 
MARK \t@N13 A@M14
COM
Problem 108. Black to play.  

In answer to the marked stone, White extends to A. What should
Black do next ? 
ENDCOM
B 1 O12
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the key point.
ENDCOM
  VAR
  B 1 N12
  COM
  Failure.

  Black 1 is ineffective.
  ENDCOM
  W 2 P11
  COM
  Even though the sequence from White 2 ...
  ENDCOM
  B 3 Q10
  W 4 P10
  B 5 Q9
  W 6 P9
  B 7 Q8
  COM
  ... Black 7 gives Black profit on the right side, ...
  ENDCOM
  W 8 L13
  COM
  ... Blacks two stones in the center are under attack when White
  plays 8. Comparing this diagram with the Correct Answer, it should
  be clear that this result is unsatisfactory for Black.
  <=
  ENDCOM
  ENDVAR
W 2 S13
COM
In answer to Black 1, White must defend her corner territory
with 2.
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  VAR
  W 2 N12
  COM
  Suppose White subdues this stone with 2 ...
  ENDCOM
  B 3 M13
  W 4 O11
  COM
  ... and 4.
  ENDCOM
  B 5 R15
  COM
  Black would then cut through with 5 ...
  ENDCOM
  W 6 S15
  B 7 S14
  COM
  ... and 7, ...
  ENDCOM
  W 8 S13
  B 9 R13
  MARK A@Q13
  COM
  ... then atari at 9. White can't play at A because she is short
  of liberties, so ...
  ENDCOM
  W 10 T14
  COM
  ... she has to capture with 10. Next, ...
  ENDCOM
  B 11 Q13
  COM
  ... Black ataris with 11 ...
  ENDCOM
  W 12 P12
  B 13 P11
  COM
  ... and 13, and ...
  ENDCOM
  W 14 O12
  B 15 M12
  W 16 O10
  B 17 S14
  COM
  ... and the sequence continues up to Black 17. White ends up
  with a weak group in the center and two weak stones at the top.
  Black has a huge advantage.
  <=
  ENDCOM
  ENDVAR
B 3 N12
W 4 L13
B 5 L11
COM
Black chases the group at the top with 5, making influence in
the center.
ENDCOM
W 6 J13
B 7 K16
COM
He then defends the top with 7. Black should be quite satisfied
with this result.
<=
ENDCOM