EVENT
BOARDSIZE 19
SETUP W D16 D10 D4 O17 L18 O15 Q18 
SETUP B Q4 Q10 Q16 K4 L16 Q14 R17 
MARK 1@O17 2@L16 3@O15 4@Q14 5@Q18 6@R17 7@L18
COM
Problem 14. Black to play.  

In this sanren-sei opening, Black makes a high two-space pincer
with 2. The joseki to White 7 follows. Where should Black play
next ? 
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B 1 O13
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Capping with Black 1 is absolutely essential. Black has now transformed
his sanren-sei formation into a large-scale moyo.
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  VAR
  B 1 F3
  COM
  Failure.

  Clearly, Black 1 is a good point locally, but ...
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  W 2 O13
  MARK B@Q12 A@Q6 1@F3
  COM
  ... White will take the vital point of 2, and Black's stone at
  the top is drifting in the shadow of White's influence. For the
  same reason, Black 1 at A is also bad. After 2, White can aim
  at the invasion at B.
  <=
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  ENDVAR
W 2 F3
MARK \t@O13
COM
Black's aim.

If White doesn't respond to the marked stone but plays 2 at the
bottom, ...
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B 3 N16
COM
... Black can force with 3, ...
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W 4 O16
B 5 M13
MARK A@K18
COM
... then jump to 5. Black can next aim at A. Black's sanren-sei
strategy is now a success.
<=
ENDCOM