EVENT
BOARDSIZE 19
SETUP W D18 F17 H16 K16 M17 R14 Q14 R11 
SETUP B C17 D16 C14 N18 O16 Q16 R15 
MARK \t@N18
COM
Problem 145. White to play.  

The marked black stone does not adequately protect the corner.
White still has a way to invade.  
ENDCOM
W 1 R17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

First White invades at the 3-3 point, ...
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B 2 S15
  VAR
  B 2 Q17
  COM
  Variation.

  If Black 2 here, ...
  ENDCOM
  W 3 S15
  COM
  ... White hanes at 3.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 2 R16
  W 3 P18
  B 4 N17
  W 5 Q17
  B 6 P17
  W 7 S18
  ENDVAR
W 3 P18
COM
... then slides to 3, threatening to link up to the white stones
on the left.
ENDCOM
B 4 N17
COM
When Black defends at 4, ...
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W 5 S18
COM
... White lives with 5.
<=
ENDCOM