(;
AB[pb][rb][qc][rc]
AW[mb][nc][oc][pc][qd][rd][pe]
C[White plays.

Source:
    Qi Jing Zhong Miao #205, an ancient collection of mostly kyu level life and death problems.
    This problem is startlingly subtle for such a simple position.

Analysis:
    Myron Souris]
(;
W[sc]
C[Correct.]
(;
B[sb];W[pa];B[ob];W[ra];B[qa]
LB[sd:a][re:b][se:c][sf:d][rf:e]
C[White has a direct ko, but at least Black has some internal ko threats, viz., the first at A (to which White responds with B), the second at C (to which White responds at D), and then the last at E.]
)
    (;
    B[qa]
    C[A mistake.];
    W[sb];B[ob];W[oa]
    C[Black dies.]
    )
)
    (;
    W[pa]
    C[The wrong move order!]
    (;
    B[ob]
    (;
    W[ra]
    (;
    B[sa]
    C[The toughest defense.]
    (;
    W[sb];B[sc]
    (;
    W[sd];B[qb]
    C[Assuming that White has more ko threats, Black eventually has to play here.];
    W[nb]
    LB[oa:a]
    C[But White only has a 2 step ko, i.e., White has to make an approach move at A before the ko becomes direct.]
    )
        (;
        W[qb]
        C[A mistake.];
        B[qa]
        C[Black lives with a double ko!]
        )
    )
        (;
        W[qa]
        C[Fails];
        B[sc];W[sd];B[oa];W[sb];B[qb]
        C[Black lives]
        )
    )
        (;
        B[qa]
        C[Inferior];
        W[qb]
        (;
        B[oa];W[sc]
        LB[sb:a]
        C[Or A.];
        B[qa];W[sb]
        C[White has a direct ko.]
        )
            (;
            B[sc];W[sd]
            )
        )
        (;
        B[oa]
        C[Inferior];
        W[sc];B[qa];W[sb]
        C[White has a direct ko.]
        )
    )
        (;
        W[sc]
        C[A mistake.  White would like to transpose into the main line's direct ko.];
        B[qa]
        C[But Black has this saving play.];
        W[sb];B[ra]
        C[Black lives.]
        )
    )
        (;
        B[oa]
        C[A mistake.];
        W[qa];B[ob];W[sc]
        C[Black dies]
        )
    )
    (;
    W[qa]
    C[A mistake.];
    B[ra]
    C[And Black lives easily.]
    )
)

