(;
AB[pb][pc][oc][nc][mc][oe][mf][lf][kf][ke][ie][ge][gc][fd]
AW[ic][kd][lc][le][md][mb][nb][ob]
C[Black plays.

Source:
    From the 14th century Xuan Xuan QiJing problem collection.];
B[kb]
C[OK, the first move is obvious.];
W[jb];B[ib]
C[The key move.]
(;
W[ka]
C[The toughest defense.];
B[jc];W[ia]
C[White has to fight a ko to live.]
)
    (;
    W[hb]
    C[Mistake.  White can capture the stone for eye-space, but ...];
    B[jc]
    C[... Black has this killing forcing move.]
    (;
    W[kc];B[jd];W[ia];B[lb]
    C[White collapses.]
    )
        (;
        W[jd];B[ja];W[hc];B[lb];W[kc];B[jb]
        C[Black wins the semeai.]
        )
    )
    (;
    W[jc]
    C[Connecting here seems to give White enough eye-space on the inside, but ...];
    B[lb];W[la];B[oa]
    C[... Black can squeeze from the outside.];
    W[kc]
    C[White captures the 2 stones, but ...];
    B[hb]
    C[... after Black saves the stone to the left, White only has one eye.]
    )
)

