EVENT
BOARDSIZE 19
SETUP W D16 F17 C5 C4 D3 F3 O17 O18 Q17 R17 S17 
SETUP B C14 D10 C6 D5 D6 Q4 Q10 Q16 R16 P16 O16 M17 
COM
Problem 156. White to play.  

Black has made a moyo on the right with his sanren-sei formation,
but there is still a weak point in his position. Where should
White play next ? 
ENDCOM
W 1 N16
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Cutting through the black position with 1 ...
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  VAR
  W 1 O3
  COM
  Failure.

  If White approaches with 1 at the bottom, ...
  ENDCOM
  B 2 Q6
  COM
  ... Black simply answers with 2.
  ENDCOM
  W 3 L4
  COM
  White completes the sequence with 3, ...
  ENDCOM
  B 4 M15
  MARK A@N16
  COM
  ... letting Black defend against the threat of White A with 4.
  Black is satisfied with this result.
  <=
  ENDCOM
  ENDVAR
B 2 N17
W 3 N15
COM
... and 3 is the standard follow-up to this joseki.
ENDCOM
B 4 K16
W 5 N13
B 6 Q13
W 7 K14
MARK \t@F17
COM
It is particularly effective here because White has the marked
stone in place, making White 7 a severe move.
<=
ENDCOM