(;AB[ao]AB[bo]AB[co]AB[do]AB[eo]AB[ep]AB[eq]AB[dq]AB[bq]AB[aq]AB[fr]AB[gr]AB[es]AW[er]AW[dr]AW[cr]AW[cq]AW[dp]AW[cp]AW[bp]AW[ap]C[b to kill]AP[goproblems]
(;B[br];W[bs];B[ar]C[];W[ds];B[cs]C[Good!RIGHT])
(;B[bs];W[br]
(;B[ds]
(;W[as];B[ar];W[as];B[ar];W[cs]C[Only a ko]))
(;B[ar];W[as];B[ar]
(;W[ds]C[Black may only get seki here]LB[aq:A])
(;W[aq];B[ds]C[(White may choose the ko instead of sente seki if he can gain from it)]))
(;B[cs];W[as];B[ar];W[as];B[ar];W[ds]
(;B[aq]
(;W[fs]C[Seki (W can exchange A1 for B1 anytime)]))
(;B[fs]C[];W[as];B[bs];W[aq]C[W may live like this or leave out A1 and keep it as a sente seki.])))
(;B[ds];W[br]
(;B[bs];W[as];B[ar];W[as];B[ar];W[cs]C[Only a ko])
(;B[cs];W[bs]MA[ar]MA[fs])
(;B[ar];W[as]MA[cs]MA[aq]))
(;B[cs]
(;W[bs])
(;W[br]))
(;B[ar];W[br]))