(;
AB[sr][rq][qq][pq][oq]
AW[rr][or][nq][np][op][pp][qp][qo][qm][lq]
C[White plays.]
PL[W]
(;
W[pr]
C[At first glance, the kill looks easy with this simple push.];
B[qr];W[rs]
(;
B[ps]
C[But Black has this tesuji to make an eye in sente at the bottom.];
W[nr];B[rp]
)
    (;
    B[qs]
    C[Blocking here is a mistake.  Black can only make one eye in gote.];
    W[rp]
    )
)
    (;
    W[rs]
    C[Descending here doesn't help either.];
    B[ps]
    C[Black again takes the vital defensive point.]
    (;
    W[nr];B[rp]
    )
        (;
        W[rp]
        C[White eliminates the 2nd eye on the right side, but ...];
        B[nr]
        C[... Black can cut off and capture a 3rd stone.];
        W[mr];B[ns];W[ms];B[os]
        )
    )
    (;
    W[qs]
    C[The only move that works!]
    (;
    B[rs]
    C[Black can force a ko, and this result is optimal for both sides.]
    )
        (;
        B[qr]
        C[Trying to avoid the ko is an overplay.];
        W[rs];B[ps];W[rp]
        (;
        B[nr]
        C[Now this cut doesn't work.];
        W[mr];B[ns];W[ms]
        C[Black has a liberty shortage.];
        B[ss];W[os]
        C[Black dies.]
        )
            (;
            B[ss];W[nr]
            C[Black can only make one eye.]
            )
        )
    )
)

