EVENT
BOARDSIZE 19
SETUP W D18 F17 J17 R10 
SETUP B C17 D16 D14 C10 N17 Q16 
COM
Problem 68. White to play.  

Black has stones on both the left and the right. Where should
White invade ? 
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W 1 R17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White 1 is good timing.
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B 2 R16
MARK \t@R10
COM
If Black 2, his thickness facing the bottom is negated by White's
marked stone.
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  VAR
  B 2 Q17
  COM
  Variation.

  There are times when Black might want to block with 2, but ...
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  W 3 R16
  COM
  ... the black stones are left with a heavy shape.
  <=
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  ENDVAR
W 3 Q17
MARK C@P16 B@P17 A@S17
COM
Next, Black can play A, B, or C.
<=
ENDCOM