(;
AB[bm][cn][en][co][do][ep][fp][dq][gq][hq][iq][ir][dr][bp][ds]
AW[bk][cl][cm][bn][bo][cp][dp][cq][eq][fq][gr][hr][es][bq][fr]
C[Black plays.

Source:
    Problem #44 from the yose volume of Gokyo Seimyo, a nearly 200 year old problem set.]
(;
B[br]
C[Seems like a plausible 1st move, but ...];
W[cr];B[cs];W[ap]
C[... White easily wins the semeai in the corner.]
)
    (;
    B[er]
    C[Maybe Black can attack toward the right instead?];
    W[cr];B[fs];W[cs]
    C[Again, Black runs out of liberties.]
    )
    (;
    B[bs]
    C[Not a common tesuji, but the only move that works!]
    (;
    W[br];B[ap]
    C[Another nice tesuji.  Black offers to sacrifice the 2 stones on the left.];
    W[aq]
    LB[er:A]
    C[White could capture the stones below, e.g., starting with A, but then Black would make a big capture of the 2 stones on the left side.];
    B[er]
    C[But now that White has captured the 2 stones, Black can attack the stones on the right.]
    (;
    W[gs];B[fs]
    C[Black takes 1st in a favorable 2-step ko, i.e., White has to ignore 2 more ko threats than Black to win the ko.]
    )
        (;
        W[cr];B[fs]
        C[With White's liberty shortage, Black captures the stones to the right.]
        )
    )
        (;
        W[cr];B[ap]
        C[Black can still use the same sacrifice tesuji.]
        (;
        W[cs]
        C[White can capture the 3 stones on the bottom, but ...];
        B[an]
        C[... Black breaks through on the left side.]
        )
            (;
            W[aq]
            C[If White captures the stones on the left side, ...];
            B[cs]
            C[... Black connects to put White in a shortage of liberties.];
            W[ao]
            C[For example ...];
            B[er];W[br];B[fs];W[as]
            C[Black takes 1st in a 25 point direct ko.]
            )
        )
    )
)

