(;AB[rd]AB[qd]AB[pd]AB[oc]AB[nc]AB[nb]AB[na]AW[od]AW[oe]AW[pe]AW[qe]AW[re]AW[ob]AW[pc]AW[qc]C[Black to do his best in the corner.]AP[goproblems] (;B[rc];W[rb] (;B[qb];W[pb] (;B[pa];W[qa];B[oa];W[qb];B[sb]C[RIGHT]) (;B[oa];W[qa];B[pa];W[qb];B[sb]C[RIGHT]) (;B[ra];W[qa];B[sb];W[sa]C[Ko. Black can do better.]) (;B[qa];W[sd]) (;B[sb];W[qa] (;B[ra];W[sa]C[Ko. Black can do better.]) (;B[oa];W[sd]) (;B[sc];W[ra];B[oa];W[pa]))) (;B[pb];W[qb]TR[rc]TR[rd]TR[qd]TR[pd]C[The marked stones are captured.])) (;B[pb];W[qb]MA[pd]MA[qd]MA[rd]C[The marked stones are captured.]) (;B[rb];W[rc]C[The marked stones are captured.]MA[pd]MA[qd]MA[rd]) (;B[qb];W[rc]C[The marked stones are captured.]MA[pd]MA[rd]MA[qd]))