EVENT
BOARDSIZE 19
SETUP W K17 O17 O16 P16 
SETUP B D16 H17 P17 Q17 R16 R14 
COM
Problem 54. Black to play.  

Where should Black invade White's formation at the top ? 
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B 1 M17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the best move.
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W 2 M16
COM
If White attaches at 2, ...
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  VAR
  W 2 L16
  COM
  Failure.

  The diagonal move of 2 is not good.
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  B 3 M16
  COM
  Black pushes up with 3, ...
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  W 4 M15
  B 5 M18
  MARK B@O18 A@K18
  COM
  ... then descends to 5, enabling him to link up with A or B.
  White's stones are thin and vulnerable.
  <=
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  ENDVAR
B 3 M18
COM
... Black descends to 3 and ...
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W 4 L16
B 5 O18
MARK A@K18
COM
... he can link either at 5 or at A. White is satisfied with
her thickness.
<=
ENDCOM