EVENT
BOARDSIZE 19
SETUP W C17 D4 E3 E2 F4 F5 F6 F8 F10 K3 M4 P4 
SETUP B C3 C4 C6 H9 H7 G5 G4 G3 F3 K5 R4 R6 R17 
COM
Problem 167. White to play.  

In the lower left corner, Black and White both advanced into
the center. However, White ended in sente. How should she play
now so as take control of the game ? 
ENDCOM
W 1 C8
MARK B@C9 A@B5
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White 1 is a strong move that takes control of the left side
and threatens the black stones with a peep at A. An extension
to Black B would be a very big move.
ENDCOM
  VAR
  W 1 H11
  COM
  Failure.

  With respect to the black stones in the center, White 1 is certainly
  a big point, but Black will ignore it ...
  ENDCOM
  B 2 C9
  COM
  ... and extend to 2. There is a big difference between this diagram
  and the Correct Answer. Here, Black has expended his territory
  on the left, leaving the white stones thin.
  <=
  ENDCOM
  ENDVAR
B 2 C7
COM
Black has to defend with 2, ...
ENDCOM
W 3 D8
MARK A@B8
COM
... forcing White to strengthen her stones with 3. Instead of
3, White could also descend to A.
<=
ENDCOM
  VAR
  W 3 B8
  MARK @@D8
  COM
  Black's bad aji.

  Descending with White 3 (suppose a White stone at @) is sente
  against the black stones in the corner. If Black ignores this
  move, ...
  ENDCOM
  B 4 K10
  COM
  (Black 4 is played to get 'White to move'.)
  ENDCOM
  W 5 B2
  COM
  ... White will play 5 ...
  ENDCOM
  B 6 C2
  W 7 B5
  COM
  ... and 7, and Black's stones are dead.
  <=
  ENDCOM
  ENDVAR