(;AP[goproblems]AB[ra]AB[rb]AB[qc]AB[qd]AB[pd]AB[od]AB[nd]AB[nc]AB[nb]AW[ob]AW[oc]AW[pc]AW[qb]AW[sb]AW[rc]AW[rd]AW[qe]AW[rf]C[Black to settle the corner.](;B[pa]C[Good.RIGHT])(;B[oa];W[qa]C[White gains two stones when they could have been saved.](;B[pb];W[sa])(;B[pa];W[sa]))(;B[qa];W[pa](;B[pb]C[Ko is not necessary.])(;B[na];W[sa])(;B[oa];W[na](;B[pb])(;B[ma];W[sa])))(;B[pb];W[pa](;B[qa];W[sa])(;B[na];W[oa])(;B[oa];W[na])))