(;
AB[ob][pc][od][qd][qe][re]
AW[qc][qb][rd][sc][pb]
TR[pb][ob]
C[Black plays.]
(;
B[se]
C[Descending is often the killing move, but is bad timing here.];
W[ra]
C[White takes the vital point.];
B[rc]
(;
W[sb]
LB[pa:A][sd:B]
C[A and B are miai for White to live.]
)
    (;
    W[pa]
    C[White can also descend here.];
    B[sd];W[sb]
    LB[oa:A]
    C[White lives.  Depending on the rest of the board, Black may need to block in gote at A.]
    )
)
    (;
    B[ra]
    TR[pb][ob]
    C[This move shows how the marked exchange is bad for White.];
    W[rb];B[se]
    C[Now this descent works.]
    (;
    W[sd];B[sb]
    C[Black has a ko for the corner.]
    )
        (;
        W[sa]
        C[This throw-in no longer works.];
        B[sd]
        (;
        W[rc];B[sb]
        LB[qa:A]
        C[Black still has a ko.  White cannot live with A, because of a liberty shortage.]
        )
            (;
            W[qa]
            C[No better.];
            B[rc]
            C[Black still has a ko.]
            )
        )
    )
)

