EVENT
BOARDSIZE 19
SETUP W Q16 R13 M17 
SETUP B D16 G17 Q10 
COM
Problem 144. Black to play.  

The white formation is called 'kannon-biraki' in Japanese, meaning
double-hinged doors. Such formations are defective and need an
additional move to complete the territory they map out. Where
should Black invade ? 
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B 1 R17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.  

When faced with this formation, invading at the 3-3 point and
living in the corner with Black 1 is the biggest move.  
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  VAR
  B 1 Q17
  COM
  Failure.

  Attaching with 1 is not good.
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  W 2 R17
  B 3 P17
  W 4 R18
  MARK A@J17 3@P17 1@Q17 2@R17
  COM
  White takes the corner with 2 and 4, leaving Black's stones 1
  and 3 heavy. White can still settle her stone at the top by extending
  to A.
  <=
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  ENDVAR
W 2 Q17
COM
Of course, White blocks from the wider side with 2.  
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B 3 R16
W 4 R15
B 5 S15
W 6 S14
MARK 4@R15
COM
White 4 and 6 are the strongest moves, but ...  
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B 7 Q15
W 8 R14
B 9 Q18
W 10 P18
B 11 S18
W 12 Q19
B 13 R18
W 14 O17
B 15 S16
COM
... Black gets six points of territory after he connects with
15.  <= 
ENDCOM