(;
GM[1]
SZ[19]
AW[jc][nc]
AB[dd][hc][pd][pf]
C[Problem 1. Black to play.  

With the black stones on the left and on the right, White's three-space extension is vulnerable. Where should Black invade ? ]
;B[lc]
TE[2]
C[Correct Answer.  

Black 1 is the vital point. No matter what White does after this, Black will have no problem. ]
;W[md]
C[If White plays 2, ... ]
;B[ld]
C[... Black pushes up with 3 and, ... ]
;W[le]
C[... after 4, ... ]
;B[jd]
C[... attaches with 5. Black dominates the top. <= ]
)
)