(;CA[gb2312]AB[qd][oc][pg][og][ph][qi][qk][pk][rl][qm][ol][om][nm][mm][lm][nn][mn]
[mo][no][oo][op][nq][or][pq][qq][qp][rn][do][dn][cn][di]AW[cc][fc][dg][eg][co][cp]
[dq][fq][ln][lo][mp][mq][on][pn][pl][pm][po][pp][qo][ro][rm][rp][rq][qr][ml][nk]
[ok][pi][oh][nh][dk]CR[dk]C[*Problem extracted from "Thirty-Six Stratagems" by Xiaochun Ma

White's last move is O. 
Let's first look at the bottom right. Is white alive or not? If it is possible to kill white's group, where should black play?
]
AP[MultiGo:4.4.4]SZ[19]AB[df]MULTIGOGM[1]

(;B[pr];W[qs]C[White's 2 is the right response to 1. ];B[sr];W[rr];B[ss];W[so];B[sq]
;W[sm]CR[rn][sq][sr][ss]C[White is alive if black plays 1, helping white to create two eyes.])
(;B[rs]
(;W[qs];B[so]C[If white responds at 2, black hanes at 3 and white only has one eye.])
(;W[sr];B[so];W[sp]C[Black 2 is a vital move in this problem, leaving white no chance but to respond at 3. 
If black chose to play elsewhere first, 2 is sente for white.]
;B[qs];W[pr];B[ps];W[oq];B[pq];W[qq];B[rr];W[oq]MA[rs]LB[mr:A][lr:B]C[Correct\:

Until 11, black makes a ko in the corner.
However, if black could exchange at A & B before X, then the white group is dead.])))