(;
GM[1]
SZ[19]
AW[jd][nd][pc][oc]
AB[dd][hc][ld][qc][pd][pf]
LB[oc:3][pc:1][qc:2]
TR[ld]
C[Problem 12. Black to play.  

In response to Black's invasion with the marked stone, White plays 1 and 3. How should Black respond ? ]
(;B[qb]
TE[2]
C[Correct Answer.  

Black should simply descend to 1, taking the corner. White still cannot link up her stones. <= ]
)
(;B[lf]
C[Failure.  

Black 1 is possible, but ... ]
;W[qb]
C[... after White 2, Black suffers a loss in the corner. <= ]
)
)