This problem shows a tesuji you can often use in real games, too...
Please find the best result for Black!
(japanese rules)
(;SZ[4]AW[ba]AW[bb]AW[ac]AB[cb]AB[db]AB[dd]C[Black to move - what's the best he can do?]GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]RU[Japanese]KM[0.00]PW[White]PB[Black]
(;B[bc]
(;W[cc]C[];B[bd]
(;W[ca]
(;B[ab]C[Black has an eye, White has no eye, so... Black wins! RIGHT])
(;B[dc]C[Seki, Black wins by one point (the prisoner). There is a better solution, try to read it out.RIGHT]))
(;W[ab]
(;B[ca]C[White is short of liberties and can't atari with A1, so Black wins. RIGHT])
(;B[cd];W[ca]C[Seki, Black wins by one point (the prisoner). There is a better solution, try to read it out.RIGHT])
(;B[dc];W[ca]C[Seki, Black wins by one point (the prisoner). There is a better solution, try to read it out.RIGHT])))
(;W[bd];B[cc]
(;W[ca]
(;B[ab]C['One eye beats no eye' - Black wins!RIGHT])
(;B[da];W[ab]C[Seki
