(;C[White to play.]AB[nc]AB[nd]AB[od]AB[pd]AB[pb]AB[qc]AW[pc]AW[oc]AW[lc]AW[lb]AW[le]AW[mf]AW[of]AW[qf]AW[qe]AW[qd]AW[re]
(;W[ob];B[nb]
(;W[qb];B[rc]
(;W[pa];B[rb]
(;W[ra];B[na]
(;W[oa];B[sa];W[sb];B[sc]C[]
(;W[sd];B[sa]C[Two-stage ko in white's favor; the best result.RIGHT])
(;W[rd];B[sa]C[Two-stage ko in white's favor; the best possible resultRIGHT]))
(;W[sb];B[oa])
(;W[rd];B[oa]))
(;W[na];B[ma]
(;W[mb];B[oa]C[Direct ko, black takes first. White has a better ko.])
(;W[oa];B[mb];W[ra];B[sc];W[sb];B[sa]C[This is ko, but white has to win it twice. There is a better ko.])))
(;W[rb];B[oa]))
(;W[rc];B[qb];W[rb];B[oa])
(;W[oa];B[qb]C[Capturing four stones in a row is enough for two eyes for black.]))
(;W[qb];B[ob]
(;W[pc];B[rc];W[rb];B[sb])
(;W[rc];B[pc];W[rb];B[mb]))
(;W[rc];B[ob]
(;W[qb];B[pc];W[rb];B[mb])
(;W[pc];B[qb]
(;W[mb];B[rb];W[sb];B[nb])
(;W[rb];B[mb])))
(;W[rb];B[qb];W[rc];B[mb];W[ob];B[oa];W[qa];B[nb]TR[ma]TR[oc]C[The marked points are mi-ai for two eyes.])
(;W[nb];B[ob]
(;W[mb];B[rb])
(;W[rc];B[mb])))