EVENT
BOARDSIZE 19
SETUP W J17 O17 
SETUP B D16 G17 Q14 Q16 R11 
COM
Problem 20. Black to play.  

White has made a four-space extension along the top. Where should
Black invade ? 
ENDCOM
B 1 L17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the vital point to invade. In this case, it is even
better than A.
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  VAR
  B 1 M17
  MARK @@L17
  COM
  Failure.

  @ is even better.
  <=
  ENDCOM
  ENDVAR
W 2 J15
COM
If White jumps to 2, ...
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B 3 L15
COM
... Black follows with 3.
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W 4 N15
B 5 L13
COM
After 5, Black can attack on the left or the right.
<=
ENDCOM