EVENT
BOARDSIZE 19
SETUP W D17 Q18 R18 R17 R16 R14 R13 O17 O15 O13 D3 C6 
SETUP B E18 F16 D15 B15 P18 P17 Q17 Q16 Q15 Q14 Q13 O16 Q4 
MARK A@O16
COM
Problem 125. White to play.  

Black wedges in with A. What should White do ? 
ENDCOM
W 1 N16
MARK A@O16
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Answering Black A with the atari of White 1 is obvious, but ...
ENDCOM
B 2 N17
COM
... White must be careful when Black cuts with 2.
ENDCOM
W 3 M17
COM
White 3 ...
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  VAR
  W 3 O18
  MARK 2@N17
  COM
  Failure.

  Answering the cut of Black 2 by descending to 3 might be good
  in some situations, but here ...
  ENDCOM
  B 4 N18
  W 5 O19
  B 6 P16
  W 7 P19
  B 8 R15
  W 9 S15
  B 10 R12
  W 11 S16
  B 12 S12
  W 13 S13
  B 14 T13
  W 15 S14
  B 16 P11
  COM
  ... Black gets a good position on the outside with the sequence
  to 15.
  ENDCOM
  W 17 M17
  COM
  Just like the Correct Answer, White has a strong position in
  the top right, but here, Black's stones on the right side are
secure and work well with his stone in the lower right corner.
  <=
  ENDCOM
  ENDVAR
B 4 O18
W 5 N18
COM
... and 5 are the best moves.
ENDCOM
B 6 N15
W 7 M16
B 8 O14
W 9 N14
B 10 P15
COM
The sequence continues to Black 10.
ENDCOM
W 11 M14
MARK A@M15
COM
Here White 12 is the strongest move. White gets an excellent
result because Black has gained absolutely no territory, while
White has built a thick position on the outside. Later, if White
gets to play A, Black might have a hard time to get two eyes.
<=
ENDCOM