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BOARDSIZE 19
SETUP W K17 O17 
SETUP B D16 H17 Q16 Q14 
COM
Problem 1. Black to play.  

With the black stones on the left and on the right, White's three-space
extension is vulnerable. Where should Black invade ? 
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B 1 M17
USER
CORRECT ANSWER
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COM
Correct Answer.

Black 1 is the vital point. No matter what White does after this,
Black will have no problem.
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W 2 N16
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If White plays 2, ...
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B 3 M16
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... Black pushes up with 3 and, ...
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W 4 M15
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... after 4, ...
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B 5 K16
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... attaches with 5. Black dominates the top.
<=
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