EVENT
BOARDSIZE 19
SETUP W D17 Q17 D4 
SETUP B C15 R15 Q4 
MARK 1@C15 2@Q17
COM
Problem 85. Black to play.  

An approach move at White 2 would seem to be common sense to
prevent Black from making a corner enclosure. But White 2 is
a mistake. How should Black punish White ? 
ENDCOM
B 1 P16
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black should press White with 1 ...
ENDCOM
W 2 P17
B 3 O16
COM
... and 3, ...
ENDCOM
W 4 N17
B 5 E16
COM
... followed by 5 ...
ENDCOM
W 6 E17
B 7 F16
COM
... and 7.
ENDCOM
W 8 G17
B 9 Q10
MARK 1@P16 \t@Q17
COM
He then extends along the right side with 9.

If White wants to make an approach move, the high approach at
1 (instead of the marked stone) would give her a better result.
<=
ENDCOM
  VAR
  B 9 O17
  COM
  Overconcentrated.

  Black could also force White's stones at the top to become overconcentrated
  with the forcing sequence from 9 ...
  ENDCOM
  W 10 O18
  B 11 N18
  W 12 M18
  B 13 N16
  W 14 M17
  B 15 P18
  W 16 N19
  B 17 R17
  W 18 Q18
  B 19 F17
  W 20 F18
  B 21 G18
  W 22 H18
  B 23 G16
  W 24 H17
  B 25 E18
  W 26 G19
  B 27 C17
  COM
  ... to 27.
  ENDCOM
  W 28 D18
  B 29 Q10
  COM
  Black now has a big lead.
  <=
  ENDCOM
  ENDVAR