(;
AB[dn][en][co][fo][fp][gp][cq][dq][gq][cr][gr]
AW[fn][gn][eo][go][io][ep][eq][fq][iq][dr][er][hr]
C[Black plays

Source:
    Qi Jing Zhong Miao #305, an ancient collection of mostly kyu level life and death problems.

Side variation analysis:
    Myron Souris];
B[fs]
C[A simple but important tesuji!]
(;
W[hp];B[ds];W[es]
(;
B[dp];W[cs];B[do];W[hq];B[bs]
C[Black takes the ko 1st.]
)
    (;
    B[cs]
    C[Fails.
Black cannot do better than a ko.];
    W[fr]
    C[Now White wins the semeai outright.]
    )
)
    (;
    W[es]
    C[Fails];
    B[do];W[hp];B[dp]
    (;
    W[cs];B[fr]
    C[White loses the semeai.]
    )
        (;
        W[hq]
        C[No better.];
        B[cs]
        C[Black still wins the semeai by 1.]
        )
    )
)

