EVENT
BOARDSIZE 19
SETUP W D16 C17 F16 R14 R11 Q5 R6 Q7 C3 D5 
SETUP B H17 P17 R16 R5 R4 Q3 O3 C14 B16 C11 
COM
Problem 175. Black to play.  

There are a lot of points that Black might play, but he should
focus on strengthening his position at the top. Where should
he play ? 
ENDCOM
B 1 P15
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the best point in this position, since it prevents
White from expanding his own position on the right.
ENDCOM
  VAR
  B 1 L16
  COM
  Failure.

  Black 1 is a rather dull move.
  ENDCOM
  W 2 P15
  COM
  White expands her moyo on the right while flattening Black with
  2.
  ENDCOM
  B 3 O16
  W 4 H16
  COM
  She then defends her stones on the left with 4 ...
  ENDCOM
  B 5 J16
  W 6 G17
  MARK 1@L16
  COM
  ... and 6. Black 1 thinks only of defending his territory at
  the top.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 H15
  COM
  Failure.

  Black 1 also misses the point.
  ENDCOM
  W 2 D14
  COM
  White first forces with 2 to make sure that her stones on the
  left can escape, ...
  ENDCOM
  B 3 C13
  W 4 P15
  COM
  ... then plays the vital point of 4.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 J3
  COM
  Failure.

  The top is most important in this position. If Black plays at
  the bottom with 1, ...
  ENDCOM
  W 2 P15
  B 3 O16
  W 4 O15
  B 5 N15
  W 6 N14
  B 7 N16
  W 8 M14
  B 9 L16
  W 10 N6
  COM
  ... White will expand her moyo on the right side with the sequence
  to 10.
  <=
  ENDCOM
  ENDVAR
W 2 L16
COM
Next, if White invades at 2, ...
ENDCOM
  VAR
  W 2 H15
  MARK C@Q12 B@R13 A@Q13 1@P15
  COM
  Continuation.

  One of the aims of Black 1 is to build influence with the sequence
  Black A - White B - Black C, so it is not a good idea for White
  to invade at the top. Therefore, she might exchange 2 ...
  ENDCOM
  B 3 K16
  COM
  ... for 3, ...
  ENDCOM
  W 4 C9
  COM
  ... then extend to 4.
  ENDCOM
  B 5 H4
  COM
  Black would then take the last extension at 5.
  <=
  ENDCOM
  ENDVAR
B 3 H15
MARK 2@L16
COM
... Black will jump to 3 and the white stones on the left, as
well as the one at 2, will come under attack.
<=
ENDCOM