(;AB[pd]AB[pf]AB[nc]AB[pg]AW[qc]C[Kill the invader. Please start with one of the marked points]AW[qh]AW[jc]TR[re]TR[rd]TR[qd]AP[goproblems]
(;B[re];W[rd]C[Please choose between marked points.]TR[qe]TR[qd]TR[pc]
(;B[pc];W[pb]
(;B[ob];W[rb]C[ko])
(;B[rb];W[ob]
(;B[qb];W[nb]C[w lives part])
(;B[nb]C[White is dead. S15 is the killing move. The key here is that black will answer A with B.RIGHT]LB[qe:A]LB[rf:B])))
(;B[qe];W[ob]C[w lives at least part])
(;B[qd];W[rb]C[Ko at best -- see branch starting with R16]
(;B[pb];W[qe]
(;B[sd];W[sc]C[ko])
(;B[pe];W[qf])
(;B[rf];W[qb]
(;B[sd];W[sc]C[ko])
(;B[sc];W[pc];B[ob];W[od];B[pe];W[oc];B[nd];W[oe];B[nf];W[ne];B[me];W[of];B[ng];W[og];B[oh]
(;W[ph];B[nh])
(;W[sd];B[se];W[rc];B[qa]
(;W[ph]C[CHOICE];B[nh];W[rg];B[qf];W[qg];B[ra];W[sb];B[pa];W[nb];B[mb];W[sg];B[na];W[sf])
(;W[nb];B[mb];W[ph];B[nh];W[qg];B[qf];W[rg];B[na];W[sg];B[ra];W[sb];B[pa];W[sf])))))
(;B[pc];W[pb])))
(;B[qd];W[rd];B[re];W[rb]
(;B[pb];W[qe]
(;B[sd];W[sc]C[ko])
(;B[pe];W[qf])
(;B[rf];W[qb]
(;B[sd];W[sc]C[ko])
(;B[sc];W[pc];B[ob];W[od];B[pe];W[oc];B[nd];W[oe];B[nf];W[ne];B[me];W[of];B[ng];W[og];B[oh]
(;W[ph];B[nh])
(;W[sd];B[se];W[rc];B[qa]
(;W[ph]C[CHOICE];B[nh];W[rg];B[qf];W[qg];B[ra];W[sb];B[pa];W[nb];B[mb];W[sg];B[na];W[sf])
(;W[nb];B[mb];W[ph];B[nh];W[qg];B[qf];W[rg];B[na];W[sg];B[ra];W[sb];B[pa];W[sf])))))
(;B[pc];W[pb]))
(;B[rd];W[qd]
(;B[qe];W[rc];B[re];W[ob]
(;B[nb];W[oc];B[od];W[pc]C[ko at best...]TR[nd])
(;B[pc];W[nb]C[w lives in part]))
(;B[re];W[od]C[Please play at A]LB[oc:A];B[oc];W[pe]
(;B[pc];W[qe]C[Freedom or life]LB[ne:F]LB[qf:L])
(;B[qe];W[pc];B[ne];W[nd];B[md];W[mc]C[W lives if the ladder is good...]LB[ih:lad]LB[jh:der])
(;B[oe]C[So S16 kills if R16 can be answered with S15. An easier alternative suggested by Guo Juan is simply S15 right away.RIGHT]))))