(;
GM[1]
SZ[19]
AW[cn][cf][oi][ph][qf][qc][pc][oc][nc][od][pn][nj][oj]
AB[jp][jd][pj][dj][dp][pd][pp][dd][fd][lc][fp][nh][nd][ne][oe][pe][qd][pk][ol][ni]
LB[pn:1][ol:2][nj:3]
C[Problem 18. Black to play.  

In the game in Problem 18, instead of playing  the honest move of 3, White exchanged 1 for Black 2, then turned at 3. What should Black do now ? ]
(;B[on]
TE[2]
C[Correct Answer.  

Attaching with Black 1 is severe. ]
(;W[po];B[oo];W[qp];B[qq];W[pq];B[op];W[rq];B[qr]
(;W[rr];B[pr];W[ro]
C[White just barely lives with the sequence to 12, but the wall Black has made will have a strong effect on White's stones above. ]
;B[lj]
C[Black has sente so he starts an attack on these stones with 13. <= ]
)
(;W[ro]
C[Variation. ]
;B[rr];W[sr]
C[White plays the sequence to 12, hoping to live in sente, ... ]
;B[oq];W[lj]
C[... then jumps to 14. ]
;B[qn]
C[However, Black attaches at 15 and ... ]
;W[qm];B[pm];W[qo];B[rm];W[ql];B[rn];W[sn];B[rl];W[qk];B[rk];W[qj];B[qi];W[rj];B[ri]
C[... White's stones in the lower right die with the sequence to 29. <= ]
))
(;W[qq]
LB[on:1]
C[Variation.  

In response to Black 1, White might invade the corner with 2. ]
;B[qp]
C[Black blocks with 3, ... ]
;W[pq];B[rq];W[rr];B[rp]
LB[rq:5]
C[... then hanes and connects with 5 and 7. ]
;W[nq];B[op];W[mr]
C[White lives in the corner with the sequence to 10, but ... ]
;B[mj]
C[... Black plays 11, attacking the five stones on the right side. ]
;W[nl];B[mk];W[nk];B[lm];W[om];B[pm];W[nm];B[qn];W[nn]
C[When White plays 20, ... ]
;B[np]
C[... Black stops her from linking up to her stones at the bottom with 21. Now the white stones have no way to escape. <= ]
))
(;B[ml]
C[How the game continued.  

Black jumped to 1 and ... ]
;W[lj]
C[... White escaped into the center with 2. ]
;B[nn]
C[Black capped with 3, but ... ]
;W[qq];B[pq];W[qp]
(;B[po];W[qo];B[on];W[mc];B[ld];W[pm]
LB[np:A]
C[... White easily lived in the corner with the sequence to 12. In addition, Black still has some bad aji at A which White might be able to exploit later. <= ]
)
(;B[qr]
LB[qp:6]
C[Variation.  

After White 6, ... ]
;W[rr];B[pr]
LB[qr:7]
C[... Black could have haned and connected with 7 and 9. ]
;W[rn];B[rs];W[sr]
LB[np:C][oo:B][po:A]
C[After White 12, Black ends in sente as before, but White's territory in the corner is smaller and Black's aji at the bottom is good. If White A or B, Black will jump to C. <= ]
))
)