(;
AB[ol][rl][rm][pm][pn][po][pp][mq][nq][oq][qq][rq][mr]
AW[qn][qo][qp][rn][so][rp][pq][pr][or]
C[Black plays.]
(;
B[qr]
C[Simply descending here seems good enough.  Both White and Black's 3 stone groups have 3 liberties, but White's seems weaker by not being able to make an eye in the corner.]
(;
W[sq]
C[For example, ...];
B[rs];W[sr];B[nr]
C[Black wins the semeai.]
)
    (;
    W[sr]
    C[Also falls short.];
    B[rs]
    C[Again Black wins the semeai.]
    )
    (;
    W[rs]
    C[Unfortunately for Black, White has this tesuji, playing from the 2-1 point on the weaker side.]
    (;
    B[qs];W[sq];B[sr];W[ps]
    C[Black only has a ko.]
    )
        (;
        B[nr]
        C[If Black tries to avoid the ko, ...];
        W[sq];B[os];W[rr];B[ps];W[sr]
        C[White lives.]
        )
        (;
        B[sq]
        C[If Black tries to avoid the ko this way, ...];
        W[qs];B[nr];W[sr];B[os];W[ps]
        C[... White lives with even more territory.]
        )
    )
)
    (;
    B[rs]
    C[Correct.  Black takes the vital point first.]
    (;
    W[sq];B[qr];W[sr];B[nr];W[rr];B[qs]
    C[Black wins the semeai.]
    )
        (;
        W[qr];B[rr];W[sq];B[nr];W[sr];B[os]
        C[Again, Black wins the semeai.]
        )
    )
)

