EVENT
BOARDSIZE 19
SETUP W D17 P17 R5 
SETUP B R16 R9 Q3 C4 
MARK 1@R9 \t@R16
COM
Problem 20. White to play.  

Black neglects to reinforce his marked stone and pincers at 1.
How should White counter this mistake ? 
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W 1 Q15
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White should press Black with 1 ...
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B 2 R15
W 3 Q14
COM
... and 3.
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B 4 R13
COM
Black's position on the right side is now rather flat.
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W 5 R14
COM
White can continue with the forcing sequence from 5 ...
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B 6 S14
W 7 S13
B 8 S12
W 9 Q13
B 10 R12
W 11 S15
B 12 T13
W 13 R17
COM
... to 13.
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B 14 S16
MARK \t@R9
COM
The marked black stone is now too close to his thick position
above, so his stones are overconcentrated.
<=
ENDCOM