(;
AB[gs][gr][gq][fp][ep][dp][cp][cq][bo][br][bs]
AW[ir][ip][ho][gn][en][cn][bn][co][bp][bq][cr][cs][dq][eq][es]
C[Black plays.

Source:
     Chinese life and death pamphlet]
(;
B[aq]
C[An innocuous-looking forcing move that begins to set up a weakness in White's position.

But because Black has another first move that also works, problemists call this artistic flaw a "dual".  The next variation shows this dual.];
W[ao]
(;
B[dr];W[ds];B[er]
C[Black continues to make seemingly pointless forcing moves.];
W[fr];B[fq]
LB[fs:A]
C[A also works, but this duplicate solution isn't really the dual problem mentioned earlier.];
W[ar];B[fs];W[as];B[er]
C[Finally, White's weakness becomes apparent.];
W[dr]
C[Otherwise Black captures 2 stones to make eyes.];
B[br]
C[This throw-in sets up a snapback.]
(;
W[bs];B[br]
)
    (;
    W[fr]
    C[No better.];
    B[bs]
    C[Black still captures the corner.]
    )
)
    (;
    B[er]
    C[This move and similar moves in this area give White an extra liberty.];
    W[dr];B[fq];W[fr];B[fs];W[ar];B[er];W[as]
    C[Black has nothing.]
    )
)
    (;
    B[dr]
    C[The dual.  This forcing move and the main line move are playable in either order.];
    W[ds];B[er];W[fr];B[aq];W[ao]
    AB[mq]
    C[Transposes to the main line.]
    )
    (;
    B[ar]
    C[An interesting move that doesn't quite work.];
    W[ao];B[dr]
    C[Black can now try the same technique in the main line.];
    W[ds];B[er];W[fr];B[fq];W[aq];B[fs];W[as];B[er];W[dr];B[br]
    C[But Black has to needlessly fight a ko.]
    )
)

