Thanks Dry, Bass, Andre Engels, Sverre, and Awelux!
(;AB[ca]AB[da]AB[db]AB[cc]AB[cd]AB[ce]AB[be]AB[bf]AB[af]AW[ag]AW[bg]AW[cf]AW[cg]AW[de]AW[dd]AW[dc]AW[ea]AW[eb]AW[fc]LB[ea:1]LB[da:2]C[Black has played 2 in response to 1. What can white do?]AP[goproblems]
(;W[bb];B[cb]
(;W[ac];B[ad]
(;W[ba];B[bc])
(;W[bc];B[ba];W[aa]C[This is an unfavorable ko for white.]))
(;W[ad]
(;B[bc]C[CHOICE];W[ac];B[ba];W[ab]C[Gote seki. RIGHT])
(;B[ac];W[bc]
(;B[ba];W[ab];B[bd])
(;B[bd];W[ab];B[ba])))
(;W[bd];B[bc]
(;W[ac];B[ad])
(;W[ad];B[ab])
(;W[ab];B[ad]))
(;W[bc];B[ba]
(;W[ad]C[]
(;B[ab]C[CHOICE]
(;W[ac]C[Thousand-year ko. Check out the excellent comments on this position. :) Thanks everyone!RIGHT])
(;W[ae]C[Gote seki.RIGHT])
(;W[bd]C[Gote seki. RIGHT]))
(;B[ac];W[ab];B[bd]C[]))
(;W[ab]C[]
(;B[ad]C[CHOICE];W[bd]C[Gote seki.RIGHT])
(;B[bd];W[ad]C[Gote seki.RIGHT]))
(;W[bd];B[ad];W[ab]C[Gote seki.RIGHT])
(;W[ac];B[ad];W[aa]C[This is an unfavorable ko for white.])
(;W[aa];B[ad];W[ac]C[This is an unfavorable ko for white.])))
(;W[ba];B[cb]
(;W[bb];B[bc])
(;W[bc];B[bb];W[ab]C[Unfavorable ko for white.])
(;W[ab];B[ad];W[bc];B[bb]C[])))