(;
AB[nq][oq][mp][lp][kp][jq][jr][iq][hq][no][pq]
AW[nr][mq][lq][kq][kr][jp][ip][hp][gp][gq][eq]
C[Black plays.]
(;
B[ls]
C[The vital point.]
(;
W[hr];B[ks];W[mr];B[or];W[is];B[js];W[ms];B[gr]
C[Another must-know tesuji.  Black sacrifices a stone to stay one move ahead in the semeai.]
(;
W[os]
C[White has nothing better than to hane here.]
(;
B[ps];W[fr]
C[At best, White only has an unfavorable 2 step ko.]
)
    (;
    B[fr]
    C[As an exercise for the reader, determine if Black can do even better than a favorable 2 step ko by extending here.]
    )
)
    (;
    W[fr]
    C[White needs 2 moves to capture the sacrificed stone.];
    B[os]
    C[Black wins the semeai.]
    )
)
    (;
    W[ks]
    C[Cutting off the stone only hastens White's capture.];
    B[mr]
    )
)
    (;
    B[hr]
    C[Normally a good shape move, but here Black must win the semeai, not just look good!];
    W[is]
    C[Now White takes Black's corresponding vital point.];
    B[js];W[ks]
    C[Black is behind by a move in the semeai.]
    )
)

