(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PB[Noir]AW[ch][cj][ek][bl][cl][dm][dn][do][dp][ep][fp][gp][hp][ip][dq][iq][hr][jr]AB[dl][el][fl][bm][cm][cn][fn][hn][jn][co][jo][cp][jp][cq][eq][fq][gq][hq][kq][dr][kr]TR[dl][el]
;B[]
;W[er]C[White neglected the ko threat (the marked stones) and must now resurrect his group by means of a favorable ko]
;B[fr]
;W[cr]
;B[es]
;W[br]
(;B[gs]
;W[bq]C[threatening the left side]
(;B[bp]
;W[cs]C[theratening a direct ko]
(;B[ds]
;W[aq]C[threatening life]
;B[as]
(;W[ap]
(;B[an]
;W[gr]
;B[hs]C[winning a liberty (and leaving a ko)]
;W[ir]
;B[ao]
;W[is]
;B[bs]
;W[ar]
;B[bs]
;W[fs]
;B[gs]
;W[hs]
;B[as]
;W[fs]C[Ko; black must find the first threat]
(;B[dk]
;W[dj]
;B[gs]C[moreover, white has intrinsic threats]
;W[cr]
;B[fs]
;W[br])
(;B[er]
;W[cr]
;B[gs]C[Now, black can no longer fight the ko
]
;W[br]
;B[bq]
;W[ar]
;B[aq]
;W[cs]C[no more ko]))
(;B[ao]
;W[an]
;B[bn]
;W[bo]C[another simple ko, which costs alittle more to black]))
(;W[gr]C[If white doesnt push, it works about same, but white loses two intrinsic ko threats ]
;B[hs]
;W[ir]
;B[ap]
;W[is]
;B[bs]
;W[ar]
;B[bs]
;W[fs]
;B[gs]
;W[hs]
;B[as]
;W[fs]C[now, black can connect]
;B[er]
;W[cr]
;B[gs]
;W[br]C[but white, while no longer able to crush, has an intrinsic ko threat]
;B[ar]
;W[fs]C[hard to say which of those sequences is *the* solution]))
(;B[hs]
;W[ds]
;B[aq]
;W[er]
;B[js]
;W[ir]
;B[is]
;W[fs]
;B[dk]
;W[dj]
;B[es]
;W[dr]
;B[ks]
;W[ar]C[blanc ne peut pas perdre]))
(;B[hs]
;W[bp]
;B[bo]
;W[an]
;B[ao]
;W[am]))
(;B[bq]
;W[gs]
(;B[fs]
;W[ds]
;B[cs]
;W[bs]
;B[ar]
;W[ds]C[this sequence is probably best for black, as he gets a double-stage  ko (nidan ko)]
;B[ck]
;W[er]C[black can now fight  on the upper floor])
(;B[ar]C[the worst answer]
;W[fs]
;B[cs]
;W[gr]C[white lives without ko, thanks to the thieves' play])))