(;
GM[1]
SZ[19]
AW[dd][dg][cn][dp][jp][nq][no][nr]
AB[nm][oq][pp][pn][qj][qd][oc][fc][id]
LB[nr:2][oq:1][nm:3]
C[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 ? ]
(;W[oe]
LB[pd:B][nc:A]

TE[2]
C[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. <= ]
)
(;W[nd]
C[Not applicable here.  

White plays 1 when she wants to erase the top. ]
;B[nc]
C[In this case, Black would push with 2, ... ]
;W[ld];B[nf]
LB[nd:1]
C[... 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. <= ]
)
(;W[qh]
C[The worst move.  

Invading with 1 gibes Black a target to attack. ]
;B[qf]
C[Black first pincers with 2, ... ]
;W[oh];B[mh]
C[... then caps with 4. Black is strong above and below, so it will be hard for White to escape. <= ]
)
(;W[qe]
C[A heavy shape.  

If White plays 1 ... ]
;B[pe];W[qf]
LB[pe:2]
C[... and 3, she is left with a heavy shape ... ]
;B[rd]
LB[pe:2]
TR[qj]
C[... 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. <= ]
)
(;W[dc]
C[An iron rule.  

Locally, White 1 is a good move, but ... ]
;B[oe]
C[... 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.' <= ]
)
)