EVENT
BOARDSIZE 19
SETUP W Q4 Q12 J17 L17 M17 N17 P17 Q17 
SETUP B C16 D3 R15 P16 O15 M16 L16 K16 
MARK \t@Q12
COM
Problem 106. Black to play.  

The marked white stone is in the shadow of Black's thick wall
at the top. How should Black attack it ? 
ENDCOM
B 1 Q9
MARK \t@Q12
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black doesn't want to pincer too severely because White might
decide to sacrifice her marked stone. Therefore, Black plays
a two-space pincer with 1.
ENDCOM
  VAR
  B 1 R6
  MARK @@Q9 A@O4
  COM
  Failure.

  When Black plays the approach move of 1, he thinks that White
  is going to respond with A, enabling him to pincer the white
  stone by extending to @. But this is too much to expect.
  ENDCOM
  W 2 R8
  COM
  White would pincer at 2 instead, ...
  ENDCOM
  B 3 R3
  COM
  ... and Black has no other alternative but to invade at 3.
  ENDCOM
  W 4 R4
  B 5 Q3
  W 6 P4
  B 7 S4
  W 8 S5
  B 9 S3
  W 10 R5
  B 11 O3
  COM
  With the joseki to 11, White has made thickness at the bottom,
  effectively neutralising Black's wall at the top.
  <=
  ENDCOM
  ENDVAR
W 2 O11
COM
If White plays 2, ...
ENDCOM
B 3 R6
COM
... Black approaches with 3.
ENDCOM
W 4 R5
B 5 Q6
W 6 O4
B 7 O9
W 8 M11
B 9 H15
COM
With the sequence to 9, Black is making good use of his thickness
at the top.
<=
ENDCOM