(;
AB[bb][bc][dc][cd][dd][de][cf][ef][bg][cg]
AW[ba][cb][db][cc][ec][fc][bd][be][ce][bf]
C[Black plays.

Source:
    Qi Jing Zhong Miao #331, an ancient collection of mostly kyu level life and death problems.

Analysis:
    Myron Souris

None of the "official" solution sets in books and the Internet seem to give the optimal play!];
B[ab]
(;
W[ae]
C[So far so good.]
(;
B[ag]
C[An inferior follow up, but at least 2 Internet problem sites give this line!];
W[ac]
C[Black takes first in this direct ko, which is worth about 23 points.  But Black can do better!]
)
    (;
    B[ac]
    C[I haven't been able to find any "solution" showing this optimal line for Black!]
    (;
    W[ag]
    C[At first glance, White seems to have greatly increased the size of a direct ko by threatening to spill out down the left side.]
    (;
    B[bh]
    (;
    W[eb];B[ah];W[ca]
    C[White only has a disadvantageous 2-step ko!];
    B[jj]
    C[For example, assume that Black takes the ko, then White re-takes the ko, and then Black makes this ko threat.];
    W[aa]
    C[White has to ignore Black's ko threat to make this approach move on the corner point.  But only now does the ko become direct!];
    B[af]
    C[For example, ...];
    W[ad]
    C[... if White tries to end the ko, ...];
    B[bc]
    C[Black captures the stones AND has already made an unanswered move elsewhere.

But to win this ko, White would have had to have given Black THREE moves elsewhere!]
    )
        (;
        W[ca]
        C[No better than the optimal line.];
        B[eb]
        C[Black essentially transposes into the optimal line.]
        )
    )
        (;
        B[ah]
        C[Black might even dream of entering the Twilight Zone of 3-step kos, by starting the ko immediately!];
        W[bh]
        C[White expands the size of the ko enormously, but Black has internal ko threats in crawling down the left side.

However, even if White does not have enough ko threats, depending on what is in the lower left, White could build a 2nd line wall that could become worth a lot of points!

Very interesting!]
        )
    )
        (;
        W[ca]
        C[Slack];
        B[ag]
        C[Now Black has the option of simply capturing White's side stones instead of transposing into the advantageous 2-step ko. ];
        W[aa];B[af];W[ad];B[bc]
        C[Black has an outright gain without fighting any kind of ko.]
        )
    )
)
    (;
    W[ca];B[eb]
    (;
    W[fb]
    C[The book "Theory and Practice of Go" (and other solution sets) give this inferior line!];
    B[af]
    C[Black unconditionally captures the 4 White stones.]
    )
        (;
        W[ae]
        C[Even here, White still has a chance to transpose into the disadvantageous 2-step ko!]
        )
    )
    (;
    W[ad]
    C[Worst.  Some Internet solutions only give this weak defense.];
    B[af];W[ca];B[eb];W[fb];B[ag]
    C[Black wins the semeai.]
    )
)

