This position is inspired from a rengo game on KGS between "a1(8k)", "Uizado(18k)", "TueurNe(9k)", "Frog27(17k)".
UPDATED - Thanks to Spica for the help.
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PW[Blanc]PB[Noir]AW[gb][bc][ec][fc][gc][cd][dd][ae][be][ee][ge][df][ef]AB[bb][cb][eb][fb][cc][dc][ed][fd][bf][cf][bg]PL[W]
(;W[da]CR[da]
(;B[ac]CR[ac]C[CHOICE]
(;W[bd]CR[bd]
;B[ca]TR[ba][da]CR[ca]C[The black group is alive ...
It have a eye in B19 and another one in D19.])
(;W[ad]CR[ad]
(;B[ab]CR[ab]C[CHOICE]
(;W[bd]CR[bd]
;B[ca]TR[ba][da]CR[ca]C[The black group is alive ...
It have a eye in B19 and another one in D19.])
(;W[fa]CR[fa]
;B[ce]CR[ce]
;W[de]CR[de]
;B[bd]CR[bd]C[RIGHT ko to fight ..]))
(;B[bd]CR[bd]C[RIGHT CHOICE White have now to win the ko.])))
(;B[ca]CR[ca]C[CHOICE]
;W[ab]CR[ab]C[RIGHT The black group is dead.])
(;B[ab]CR[ab]C[CHOICE]
;W[fa]CR[fa]C[RIGHT The black group is dead, the stone in D19 cannot be eaten.]))
(;W[ab]CR[ab]
;B[da]CR[da]
(;W[fa]CR[fa]
;B[ba]CR[ba]C[The black group is alive.])
(;W[ba]CR[ba]
;B[fa]CR[fa]C[The black group is alive])))