EVENT
BOARDSIZE 19
SETUP W E4 F3 D17 C14 P17 M4 N4 O4 P4 R5 
SETUP B C4 D3 E3 F4 R16 Q15 N3 O3 Q3 P3 
MARK \t@M4
COM
Problem 98. Black to play.  

White has just extended with the marked stone, making a thick
wall at the bottom. How should Black answer this move ? 
ENDCOM
B 1 R11
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black must extend down the right side to neutralise White's thickness,
but because White is so thick below Black should extend only
as far as 1.
ENDCOM
  VAR
  B 1 R10
  COM
  Failure.

  Extending one line farther with 1 leaves Black vulnerable ...
  ENDCOM
  W 2 Q13
  MARK A@R12
  COM
  ... to an invasion at 2 or A. White may not make this invasion
  immediately, but Black must always worry about it.
  ENDCOM
  B 3 R7
  COM
  Black could settle his stones with 3, but White will use her
  thickness below to attack.
  <=
  ENDCOM
  ENDVAR
W 2 Q13
COM
Next, if White 2, ...
ENDCOM
  VAR
  W 2 R13
  COM
  If White 2, ...
  ENDCOM
  B 3 Q13
  COM
  ... Black plays 3.
  <=
  ENDCOM
  ENDVAR
B 3 R13
COM
... Black can defend at 3.
<=
ENDCOM