(;
GM[1]
SZ[19]
AW[fq][lp][pm][nj][qf]
AB[jp][jd][pj][dj][dp][pd][pp][dd][dn][np][nf][nd]
TR[lp]
C[Problem 12. Black to play.  

White invades the bottom with her marked stone. How should Black answer ? ]
(;B[nn]
TE[2]
C[Correct Answer.  

Black should jump to 1, threatening to pull out the black stone an the right and keeping the white stone on the left isolated. ]
;W[ol]
C[If White defends with 2, ... ]
;B[ko]
C[... Black 3 all but captures the white stone at the bottom right. ]
(;W[jq]
C[If White tries to make sabaki with 4 ... ]
;B[iq];W[ip]
C[... and 6, ... ]
;B[hq]
C[... White has no follow-up after Black extends to 7. <= ]
)
(;W[lr]
C[If White jumps down to the second line with 4, ... ]
;B[jr]
C[... Black blocks from the left with 5. ]
;W[oq];B[pq];W[op];B[oo];W[nq];B[pr];W[mp];B[no];W[jq];B[iq];W[kq];B[ip];W[kr];B[ir];W[ns]
LB[nn:1]
C[With the black stone at 1 present, White just barely lives with the sequence to 20, but ... ]
;B[dq]
C[... Black takes an unassailable lead when he encloses the lower left corner with 21. <= ]
))
(;B[lo]
C[How the game continued.  

Black attached with 1 ... ]
;W[mo];B[mp];W[ko];B[lq];W[ln];B[kp]
LB[ln:6]
C[... and linked up with the sequence to 7, but White made a ponnuki with 6, which gave him tremendous power in the center. ]
;W[ch]
C[White ended in sente, so she switched to the left side with 8. See Problem 16 for the continuation of this game. <= ]
)
)