EVENT
BOARDSIZE 19
SETUP W J17 O16 O17 
SETUP B D16 G17 P17 Q16 Q14 
COM
Problem 21. Black to play.  

Even when White has the two stones on the right, the four-space
extension is a bit wide. Where does Black invade ? 
ENDCOM
B 1 M16
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 shows good go sense.
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  VAR
  B 1 M17
  USER
  CORRECT ANSWER
  ENDUSER
  COM
  Correct Answer.

  Black could also invade deeply with 1.
  ENDCOM
  W 2 L16
  B 3 N15
  W 4 N16
  MARK 2@L16
  COM
  However, White can confine Black to the side with 2 and 4.
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  B 5 M16
  W 6 M15
  COM
  This will lead to an all-out fight.
  <=
  ENDCOM
  ENDVAR
W 2 M18
COM
If White tries to link up her stones with 2, ...
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B 3 L17
W 4 L18
B 5 K17
W 6 K18
B 7 J16
COM
... Black presses her in a low position with the moves to 7.
<=
ENDCOM