(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[9]KM[0.00]
PB[Noir]AW[bf][bg][cg][dg][eh][di]AB[cd][be][cf][df][ef][eg][fg][fh][fi]
(;B[dh]C[La clé ; (any other move leaves a ko or worse]
(;W[ch]C[best defense]
;B[bi]
;W[bh]
(;B[ei]C[forced:]
;W[af]
;B[ah]
;W[dh]C[not a seki : White is dead (bent four. reminder:]
;B[ae]
;W[]
;B[ai]
;W[]
;B[ag]
;W[ci]
;B[ah]
;W[ai]
;B[bi]C[the game is finished, therefore White has no threats.])
(;B[af]
;W[ei]
;B[ah]
;W[ag]C[White lives]))
(;W[bi]
;B[ch])
(;W[ah]
;B[ch]
;W[bi]
;B[af]))
(;B[af]
(;W[bi]
;B[ah]
;W[ag]
(;B[bh]
;W[ch]
;B[ei]C[ko, good for Black])
(;B[ch]
;W[bh]))
(;W[ch]
;B[bi]C[White is dead]
(;W[ei]
;B[ci])
(;W[ag]
;B[ei])))
(;B[ei]
;W[dh]C[see problem  6 : indirect ko]
;B[bh]
;W[ah]
;B[bi]
;W[af]
;B[ai])
(;B[bi]C[actually, this move lets live]
;W[bh]
(;B[dh]C[too late]
;W[ci]
(;B[ei]
;W[ai])
(;B[af]
;W[ai]
(;B[ei]
;W[ag])
(;B[ag]
;W[ei])))
(;B[af]
(;W[ci]
;B[dh]
;W[ai]C[White is alive])
(;W[ag]
;B[ci]
;W[ai]
;B[ah]
;W[ae]
;B[ei]
;W[dh]C[a very indirect ko])))
(;B[ah]
;W[bi]C[White lives]))
