(;GM[1]FF[4]AP[Drago:4.22.03]SZ[19]CA[UTF-8]AB[rs][rr][rq][ro][qn][pn][on][no][lo][mq][mr][kr]AW[nr][oq][pq][qq][qr][qp][po][op]C[Black to play and kill.]
(;B[os]C[Incorrect.]
(;W[ps];B[qs];W[or]
;B[pr]C[This allows White a Ko to live.])
(;W[or]C[White dies unconditionally if here.];B[ps])
(;W[ns]C[Also dies unconditionally if here.];B[qs];W[ps];B[pr]))
(;B[ps]C[Correct.];W[qs];B[ns];W[os]
;B[or]C[This gives atari on the one stone so Black can escape one way or the other to leave White dead.]
(;W[nq];B[os])
(;W[pr];B[nq];W[os];B[nr])))
