(;
AB[jo][lo][mo][np][op][pp][kq][lq][pq][pr][qr][ls][qs]
AW[nn][pn][no][qo][mp][qp][mq][nq][oq][qq][or][rr][os]
C[Black plays.

Note:  Hugh Zhang 7d found a cook for this problem!  Black was supposed to have a ko, but Hugh found an unconditional way for White to capture the corner stones.  Good job Hugh!  See the details in the variations.]
(;
B[mr]
C[Bad timing.

Ed.:  actually, Black cannot save the corner stones, so this move is as good as any other.];
W[rs]
C[White simplifies the semeai to win easily.];
B[lp];W[po];B[ns];W[oo]
)
    (;
    B[rs]
    C[Correct and fairly obvious.]
    (;
    W[sr]
    C[But this move is a slack defense.

Ed.:  correction.  This move lets White capture the corner unconditionally.];
    B[rq]
    C[A standard corner tesuji in many corner semeai.  Now White has to spend 2 moves to capture 1 stone.]
    (;
    W[rp]
    C[Ed.:  trying to capture this stone is a flaw in the original solution.  The variation shows how White can do better!];
    B[mr];W[sq];B[lp];W[po];B[nr];W[oo];B[ns]
    C[With the extra move, Black wins the semeai.]
    )
        (;
        W[oo]
        C[Ed.:  Hugh Zhang's improvement for White!];
        B[sq];W[rp];B[ss];W[ps];B[mr];W[po];B[nr];W[rr];B[sr];W[sp]
        C[White captures the corner unconditionally!]
        )
    )
        (;
        W[sq]
        C[The best defense!

Ed.:  actually, this conclusion is incorrect.  White has better than this ko.];
        B[mr];W[po];B[lp];W[oo];B[nr];W[ss]
        C[White can atari with this ko!  This result is locally optimal for both sides.

Ed.:  correction.  White can do better!  See previous variation.]
        )
    )
)

