EVENT
BOARDSIZE 19
SETUP W C15 D16 D14 E14 C13 C6 D4 S2 S1 R3 R4 R6 S6 P10 Q10 R10 K17 N17 R13 P13 
SETUP B C17 D17 F17 C14 D13 F3 K4 R2 Q2 Q3 Q4 Q5 Q6 R7 R8 P8 Q11 Q16 R15 O16 
COM
Problem 161. Black to play.  

Black has made a moyo in the bottom that may need some tending
to, but his three stones in the upper right corner are under
siege. What should Black do ? 
ENDCOM
B 1 F5
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black must complete his moyo at the bottom by jumping to 1. The
black stones at the top are not in great danger.
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  VAR
  B 1 O17
  COM
  Failure.

  If Black defends the upper right corner with 1 ...
  ENDCOM
  W 2 O18
  B 3 P18
  COM
  ...and 3, ...
  ENDCOM
  W 4 H3
  COM
  ... White will invade with 4 and ...
  ENDCOM
  B 5 H4
  W 6 J3
  B 7 J4
  W 8 K3
  B 9 L3
  W 10 G3
  B 11 G4
  W 12 E3
  COM
  ... wipe out most of Black's moyo with the sequence to 12. This
  is a big loss for Black.
  <=
  ENDCOM
  ENDVAR
W 2 P18
COM
The best White can do is to play 2 ...
ENDCOM
B 3 Q18
W 4 P17
COM
... and 4, ...
ENDCOM
B 5 Q17
MARK 3@Q18
COM
... but Black secures a large corner with 3 and 5.
<=
ENDCOM