(;AB[cb]AB[bc]AB[bd]AB[be]AB[aa]AW[ba]AW[da]AW[db]AW[dc]AW[dd]AW[ce]AW[cf]AW[bf]AW[bh]C[Optimal play for black?]AW[cd]AP[goproblems]
(;B[ca];W[ab]
(;B[bb]
(;W[ae]
(;B[ac]C[This is the only way for black to live without ko.RIGHT])
(;B[ad];W[aa]C[black dies.])
(;B[aa]C[Not optimal: black lives, but white has a ko threat at B19.]))
(;W[aa];B[ae];W[ac];B[ad];W[ab];B[aa];W[ba];B[ac]C[RIGHT]))
(;B[ae];W[bb];B[cc];W[ac];B[ad];W[af];B[aa];W[ab]C[ko.])
(;B[ac];W[bb]
(;B[aa];W[bb]
(;B[ae];W[cc]C[])
(;B[cc];W[ae])
(;B[ba];W[ae])
(;B[ab];W[ae]))
(;B[cc];W[ae]C[black dies.])))
(;B[bb];W[ca]C[black is dead.])
(;B[ae];W[ca]C[black is dead.]))