(;AB[ac]AB[ab]AW[ad]AW[bd]AW[cd]AW[cc]AW[bb]AW[ca]AW[cb]AB[ae]AB[be]AB[ce]AB[de]AB[dd]AB[dc]AB[db]AB[da]C[Black to kill.]AP[goproblems]
(;B[bc];W[aa])
(;B[ba];W[aa]C[Ko, white takes first. You can do better.])
(;B[aa]C[Right! This isn't a seki; it's bent-four in the corner. After removing all white's ko threats, black can kill at B19.RIGHT]))