(;AB[sr][rr][qq][pq][oq][nq][nr]AW[sq][rq][qp][ro][pp][op][np][mq][mr][ns][lr]C[Black plays.]SZ[19]

(;B[os]C[A big mistake.];W[qr]
(;B[qs];W[pr];B[ms];W[rs]C[Black collapses.])
(;B[pr]C[No better.];W[qs]C[Black cannot atari from either side.]))
(;B[qs]C[Normally a good eye shape move, but not here.];W[ps]C[White makes one of the eyes false, and ...];B[os];W[or]C[... Black cannot atari to live, because of a liberty shortage.])
(;B[ps]C[Correct.]
(;W[os]
(;B[or]C[Too greedy.];W[qr];B[ms];W[rs]C[Black dies.])
(;B[pr]C[Slack.  Black lives, but doesn't make the maximum endgame territory.

Black only has 3 points, because after White fills the outside liberties, Black must add a stone inside to defend the corner.])
(;B[qs]C[Best.  Black lives and makes the maximum endgame points.  Black can likely expect 4 points of territory.]
(;W[or]C[White can reduce the corner to only 3 points, but ...];B[pr]C[... Black ends in sente, so White probably won't give up sente here.])
(;W[pr];B[qr]C[Black has 4 points.])
(;W[ms];B[or]C[Black has 4 points.])))
(;W[qs]C[No better.];B[qr];W[os];B[rs]C[Black can likely expect 4 points of territory.])))