EVENT
BOARDSIZE 19
SETUP W C16 E17 C5 C9 L3 O3 R11 
SETUP B C4 D5 D3 F4 Q4 Q6 Q16 
MARK B@R13 C@R9 A@K17
COM
Problem 124. Black to play.  

White has played in the middle of the right side. Should Black
ignore this move and extend to A, of should he attack White with
a move at B or C ? 
ENDCOM
B 1 R9
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Attacking the white stone with 1, driving it toward the star-point
stone above, is best.
ENDCOM
  VAR
  B 1 K17
  COM
  Failure.

  If Black extends along the top, ...
  ENDCOM
  W 2 R8
  COM
  ... White will make a two-space extension down the right, attacking
  the black stones below.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 R13
  COM
  Failure.

  Black 1 is the wrong direction.
  ENDCOM
  W 2 R8
  MARK A@R7 B@Q8
  COM
  White plays 2, aiming at Black's weak underbelly. The exchange
  of Black A - White B strengthens White, so it is not good.
  ENDCOM
  B 3 P17
  COM
  If Black defends his corner at the top, ...
  ENDCOM
  W 4 L16
  COM
  ... White makes a double-wing formation in the upper left with
  4.
  <=
  ENDCOM
  ENDVAR
W 2 R14
COM
White will extend to 2.
ENDCOM
B 3 R15
W 4 Q14
B 5 O17
COM
The sequence to 5 is a joseki.
<=
ENDCOM