(;
GM[1]
SZ[19]
AW[cm][dm][em][fm]
AB[dp][pd][pp][dd][cn][dn][en][jj]
LB[cn:2][dn:4][en:6][cm:3][dm:1][em:5][fm:7]
C[Problem 123. Black to play.  

White has extended to 7. Can Black ignore this move and play somewhere else, or does he have to defend ? ]
(;B[cg]
TE[2]
C[Correct Answer.  

Black's stones in the lower left are strong, so he should extend to 1, neutralising White's wall and limiting White's expansion to A. ]
;W[fn]
C[If White turns at 2, ... ]
;B[fp]
C[... Black defends the bottom with 3. <= ]
)
(;B[fp]
C[How the game continued.  

Black reinforces the bottom with 1, but ... ]
;W[cf]
LB[cq:A]
C[... White 2 was an ideal extension. Black had invested a lot of stones at the bottom, but the territory there was not secure, since White could invade at A. <= ]
)
)