EVENT
BOARDSIZE 19
SETUP W D16 C3 D4 D6 
SETUP B Q16 Q4 D2 F3 
MARK \t@C3
COM
Problem 49. Black to play.  

What should Black do after White plays the marked stone ? 
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B 1 J3
MARK \t@D2 \t@C3
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

After exchanging the marked stones, Black must extend to 1.
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  VAR
  B 1 Q10
  COM
  Failure.

  Rushing to make a sanren-sei with 1 is bad.
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  W 2 H3
  COM
  White 2 ...
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  B 3 F5
  W 4 H5
  COM
  ... and 4 severely attack the black stones, robbing them of a
  base.
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  B 5 F7
  COM
  After Black 5, ...
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  W 6 D9
  COM
  ... White takes profit on the left side with 6.
  <=
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  ENDVAR
W 2 D10
COM
White 2 would then be the usual move in this position and ...
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B 3 Q10
COM
... Black would make a sanren-sei on the right side with 3.
<=
ENDCOM