(;
AB[so][qp][qq][qr][rp][pr][os]
AW[rm][oo][qo][ro][pp][pq][sq][nr][or]
C[Black plays.]
(;
B[sr]
C[A common move in similar positions, but fails here.];
W[rr];B[rs];W[ss];B[sp];W[qs];B[ps];W[ns]
C[Ko.]
)
    (;
    B[sp]
    C[Correct.];
    W[rr]
    (;
    B[rs]
    C[Correct.  The variations show ways for Black to go astray.]
    (;
    W[ss];B[ps];W[ns];B[rq]
    C[Double ko.  Black lives.]
    )
        (;
        W[ps];B[sr];W[ss];B[rq];W[qs];B[rs];W[qs];B[ps]
        C[Again, double ko.]
        )
    )
        (;
        B[ps]
        C[Fails.];
        W[rs];B[rq];W[sr];B[qs];W[ns];B[ss];W[sr];B[rr];W[rs]
        C[Ko.]
        )
        (;
        B[sr]
        C[Also fails.];
        W[ss]
        (;
        B[rq];W[sn]
        C[Ko.]
        )
            (;
            B[rs];W[qs];B[ps];W[ns]
            C[Ko.]
            )
        )
    )
)

