(;
AB[eb][gb][ec][dd][de][af][bf][cf][ba]
AW[db][bc][dc][ae][be][bb]
TR[ba][bb]
C[Black plays.
Win a prize if you can find a killing sequence!

Source:
     Problem #72 from Guan Zi Pu, an ancient collection of over a 1000 near dan level life and death problems.

Analysis:
    Myron Souris

Multiple pro "solutions" to this problem claim that Black kills, and give the marked exchange as the starting moves.  (One pro's "solution" doesn't even mention White's marked stone, which is the best defense.)

However, the variations below instead seem to show that White is almost alive with a favorable 2-step ko!  Can all the pros be wrong?

If you can find a killing sequence for Black, please send your analysis to the Problem of the Week Editor at potw@usgo.org.  The earliest correct response wins a back issue of "Go World" magazine.  Be sure to include side variation analysis as appropriate.]
(;
B[da]
(;
W[aa]
C["Official" solutions give only this inferior White follow up.];
B[ab];W[cb]
C[This play is too late!];
B[ac]
C[A brilliant move that kills White after White's slack play!]
(;
W[ca];B[bd];W[aa];B[ce]
C[White doesn't even have a direct ko.]
)
    (;
    W[cd]
    C[Too slow.];
    B[ce];W[ca];B[bd]
    C[White dies.]
    )
)
    (;
    W[cb]
    C[No "official" solution gives this defense, which seems to lead to a favorable 2-step ko for White!];
    B[cd]
    C[Otherwise, White plays here to form 2 eyes.];
    W[aa]
    (;
    B[ac];W[bd];B[ab];W[ca];B[ce]
    C[Forced.  Otherwise, White lives outright.  But the ko is still indirect for Black.];
    W[jd]
    C[Represents a ko threat.]
    (;
    B[cc];W[ea];B[aa]
    C[Now the ko is direct, but White made an extra move elsewhere.  So from the start of the problem, White has a favorable 2-step ko.]
    )
        (;
        B[aa]
        C[Now the ko is direct, but White made an extra move elsewhere.  So from the start of the problem, White has a favorable 2-step ko.]
        )
    )
        (;
        B[ab]
        C[No better.];
        W[ca];B[ac];W[bd]
        C[White transposes to the previous variation.]
        )
    )
)
    (;
    B[cd]
    C[No better for Black.];
    W[aa];B[ab];W[ca];B[ac];W[bd];B[cc];W[cb];B[ce];W[jd]
    C[Represents a ko threat.];
    B[aa]
    C[Now the ko is direct, but White made an extra move elsewhere.  So overall, White has a favorable 2-step ko.]
    )
)

