(;GM[1]FF[4]SZ[19]AP[SmartGo:0.9.0.1]CA[UTF-8]GN[204exRH]AB[on][pn][qn][rn][no][np][nq][nr][ns][pp][pq][qq][os][ps]AW[po][op][oq][or][rq][pr][qr]C[Black to play and kill in ko.]
(;B[rr]C[Black can try this but fails.]
(;W[qp];B[qs]
(;W[sr]C[Playing this way, White lives.];B[rs];W[ro];B[sp];W[so];B[sq];W[pq])
(;W[pq]C[White mistake];B[qo];W[ro];B[oo];W[sp]))
(;W[rs];B[qp]
(;W[rp]C[Now whichever option Black chooses White can live.]
(;B[qo];W[sr]
(;B[qs];W[sp])
(;B[sp];W[qs]))
(;B[sr];W[qo];B[qs];W[sq];B[ss];W[rs]))
(;W[sr];B[rp];W[qs];B[sp];W[sq])))
(;B[qp]C[So it is correct for Black to start here.]
(;W[qo]
;B[rr]C[Now Black plays here.]
(;W[rs];B[rp];W[sr]
;B[so]C[This is key play that sets up the ko shape.];W[qs]
;B[sq]C[After this White must take this stone and fight the ko.])
(;W[rp]
;B[qs]C[White is unconditionally dead.]))
(;W[rp]
;B[qo]C[This way White doesn't have room to make two eyes in the corner.])))
