EVENT
BOARDSIZE 19
SETUP W C17 D15 D12 C9 G3 K3 R4 Q4 R2 Q3 O4 R18 P17 O16 
SETUP B C7 C4 E3 R3 S2 S3 S4 R6 R10 Q14 R16 N17 M16 E17 H17 
MARK \t@O16 \t@P17 \t@R18
COM
Problem 21. Black to play.  

The position in the top right arose from a basic joseki. How
should Black attack the marked stones ? 
ENDCOM
B 1 Q16
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.  

Black 1 may seem like a strange move, but it is quite effective.
ENDCOM
  VAR
  B 1 O14
  COM
  Failure.

  Black 1 is slack.
  ENDCOM
  W 2 S17
  B 3 S16
  W 4 O18
  MARK A@N15
  COM
White can secure her stones with the sequence to 4, and Black's
  territory on the right side is not as secure as it was in the
  Correct Answer. White is now threatening to keep Black's positions
  separated by moving into the center with A.
  <=
  ENDCOM
  ENDVAR
W 2 P18
COM
White 2 is an important defensive point, but ...
ENDCOM
  VAR
  W 2 S17
  COM
  If White tries to get more room in the corner with 2,
  ENDCOM
  B 3 Q18
  COM
  ... Black will attach at 3, then ...
  ENDCOM
  W 4 Q17
  B 5 R17
  W 6 P18
  B 7 S18
  W 8 Q19
  B 9 S19
  COM
  ... take the corner for himself with the sequence to 9.
  ENDCOM
  W 10 O14
  COM
  White still doesn't have two eyes, so she must escape with 10.
  ENDCOM
  B 11 N15
  COM
  After 11, ...
  ENDCOM
  W 12 O15
  B 13 Q12
  MARK A@P18
  COM
  ... Black secures the right side with 12. This result is similar
  to the previous variation (White 2 at A).
  <=
  ENDCOM
  ENDVAR
B 3 S17
W 4 O14
B 5 N15
W 6 O15
B 7 Q12
COM
... Black secures the right side with the sequence to 7.
<=
ENDCOM