Black made a mistake in the joseki, and now didn't see the danger and chose to play A. How can white punish his ignorance?
(Apologies if you've seen this simple problem before, but I didn't find it with a pattern search)
(;AB[bb]AB[bc]AB[cc]AB[db]AB[eb]AB[dd]AB[ed]AW[bd]AW[cd]AW[cf]AW[cb]AW[dc]AW[ec]AW[fc]AW[fb]AB[ef]LB[ef:A]AP[goproblems]
(;W[ca]
(;B[ba]
(;W[ac]
(;B[da];W[ab]C[That's it :)RIGHT])
(;B[ab]C[Black can't play this because...];W[aa]C[It would be self-atariRIGHT]))
(;W[ea];B[da]C[Alive])
(;W[fa];B[da]C[Alive])
(;W[da];B[ea]C[Very alive]))
(;B[ad]C[Just for studying];W[ba];B[ce];W[be];B[bf];W[de];B[ae];W[ce];B[ac]
(;W[ea]C[Maybe black can survive, but he will NOT be happy with this situation.RIGHT])
(;W[bg];B[aa];W[ab]C[As there are no big threats in the opening, white should be happy.RIGHT])))
(;W[ac];B[ca]C[]
(;W[ab];B[ea];W[ba];B[aa]C[Ko and black takes first - white can do much better])
(;W[ea];B[ab]C[Alive]))
(;W[ea];B[ca]C[Alive])
(;W[ee];B[ca]C[Killing two stones (supposing you succeed) is not a great achievement])
(;W[da];B[ca]C[Now what?])
(;W[ba];B[ca]
(;W[ea];B[aa]C[Nice try, but black lives])
(;W[ac];B[aa]C[Alive])
(;W[ab];B[ac]C[Alive])))