EVENT
BOARDSIZE 19
SETUP W D16 D13 C6 D4 K4 O3 O5 O2 
SETUP B O7 P3 Q4 Q6 R10 R16 P17 F17 J16 
MARK 2@O2 1@P3 3@O7
COM
Problem 150. White to play.  

Black maps out a huge moyo with 1 and 3. It is now urgent that
White reduces the size of this moyo. Where is the weak point
of Black's formation ? 
ENDCOM
W 1 P15
MARK B@Q16 A@O17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

When Black has a tight small-knight enclosure with extensions
on both sides, the weak point is White 1. Depending on how Black
answers, White will make sabaki by playing at A of B.
<=
ENDCOM
  VAR
  W 1 O16
  COM
  Not applicable here.

  White plays 1 when she wants to erase the top.
  ENDCOM
  B 2 O17
  COM
  In this case, Black would push with 2, ...
  ENDCOM
  W 3 M16
  B 4 O14
  MARK 1@O16
  COM
  ... then cap with 4. This move is very severe, since it not only
  attacks White's two stones, but also expands Black's moyo on
  the right. Clearly, White 1 is not a good idea in this position.
  <=
  ENDCOM
  ENDVAR
  VAR
  W 1 R12
  COM
  The worst move.

  Invading with 1 gibes Black a target to attack.
  ENDCOM
  B 2 R14
  COM
  Black first pincers with 2, ...
  ENDCOM
  W 3 P12
  B 4 N12
  COM
  ... then caps with 4. Black is strong above and below, so it
will be hard for White to escape.
  <=
  ENDCOM
  ENDVAR
  VAR
  W 1 R15
  COM
  A heavy shape.

  If White plays 1 ...
  ENDCOM
  B 2 Q15
  W 3 R14
  MARK 2@Q15
  COM
  ... and 3, she is left with a heavy shape ...
  ENDCOM
  B 4 S16
  MARK \t@R10 2@Q15
  COM
  ... after Black 2 and 4. White cannot make a full extension down
  the side because of the presence of the marked stone. This is
  a terrible result for White.
  <=
  ENDCOM
  ENDVAR
  VAR
  W 1 D17
  COM
  An iron rule.

  Locally, White 1 is a good move, but ...
  ENDCOM
  B 2 P15
  COM
  ... Black then eliminates the weak point of his moyo by jumping
  to 2. When dealing with a moyo, the iron rule is 'Never let your
  opponent perfect his moyo.'
  <=
  ENDCOM
  ENDVAR