(;
AB[go][cp][dp][ep][fp][dq][gq][dr][gr]
AW[fn][gn][bo][co][do][fo][io][bq][hq][jq][cr][hr]
C[Black plays.

Source:
    Problem #54 from the yose volume of Gokyo Seimyo, a nearly 200 year old problem set.];
B[bp]
C[OK, this move is obvious.];
W[ap];B[bs]
C[The key move!]
(;
W[br]
C[White's move here protects the corner stones, without giving Black an inside forcing move to make 2 eyes.];
B[er]
C[Normally for the "door" group, this defense (and all other defenses) fail.  But Black has primed the corner with a surprise.];
W[ds];B[fq];W[fs];B[gs];W[es]
C[White's sequence here should kill Black, but ...];
B[aq]
C[... Black begins a volley of annoying throw-ins.];
W[ar];B[cs];W[as];B[cs];W[bs];B[fr]
C[And now, Black squeezes the hapless corner stones.];
W[cs];B[cq]
C[Black catches the stones in a liberty shortage!]
)
    (;
    W[cs]
    C[White can cut off the corner stone, but ...];
    B[ds]
    C[... Black has this inside forcing move.];
    W[aq]
    C[White must save the corner stones.];
    B[gs]
    C[With the 2nd inside move, Black easily makes life.]
    )
)

