My first attempt. Very easy to solve I suppose but hard for me to settle. Thanks for comments and corrections (and to darin and BartTm who already fixed one variation).
(;SZ[19]AW[fa]AW[fb]AW[bc]AW[fc]AW[bd]AW[cd]AW[dd]AW[ed]AW[fd]AB[ea]AB[bb]AB[eb]AB[cc]AB[dc]AB[ec]LB[sa:239]GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]RU[Japanese]KM[0.00]
(;B[ab]
(;W[ac];B[ca]C[RIGHT])
(;W[aa];B[ba];W[cb];B[ca]C[RIGHT])
(;W[ba];B[ca];W[cb];B[aa]C[RIGHT];W[ac]C[alive but you gave a bonus ko threat to whiteRIGHT])
(;W[ca]C[CHOICE]
(;B[cb]C[ d19 and b19 miaiRIGHT])
(;B[ba];W[cb]C[Big mistake.])
(;B[db];W[cb]C[Big mistake.])
(;B[da];W[cb]C[Big mistake.]))
(;W[cb]C[CHOICE]
(;B[ca]
(;W[ac];B[db]C[ 3 eyes...RIGHT])
(;W[ba]C[CHOICE]
(;B[aa]C[RIGHT];W[ac]C[ alive but you gave a bonus ko threat to whiteRIGHT])
(;B[db]C[RIGHT])))
(;B[aa];W[da]C[dead])
(;B[ba];W[da]C[dead])
(;B[da];W[ca]C[?????]))
(;W[da]
(;B[ca]C[RIGHT];W[cb];B[db]C[RIGHT])
(;B[cb]C[RIGHT])
(;B[ba];W[cb]C[Big mistake.])
(;B[aa];W[cb]C[Big mistake.]))
(;W[db];B[cb]C[ no ko threatsRIGHT]))
(;B[ac]
(;W[ca]
(;B[ba];W[ab];B[cb];W[da]C[only one eye])
(;B[ab];W[cb];B[ba];W[ad]C[dead])
(;B[cb];W[da]
(;B[ab];W[ba]C[Only seki.])
(;B[ba];W[ab]C[dead.])))
(;W[ab]C[CHOICE]
(;B[aa];W[ca]C[one eye, dead.])
(;B[ca];W[ad];B[db];W[ba]C[ko, black can do better.])))
(;B[aa];W[ca]C[dead shape])
(;B[ba];W[ab]C[dead shape])
(;B[ca];W[ab]
(;B[db];W[ba]C[ko, Black can do better.])
(;B[ba];W[db]C[dead.])
(;B[aa];W[ba]C[ko, Black can do better.]))
(;B[cb];W[ab]
(;B[aa];W[da]C[dead.])
(;B[ba];W[da]C[dead]))
(;B[da];W[ab]
(;B[aa];W[ba]C[dead])
(;B[ca];W[bf]C[dead in gote, shame on you.])
(;B[ba];W[cb]C[dead])
(;B[cb];W[ba]C[dead.]))
(;B[db];W[ca]
(;B[ab];W[ba]C[dead])
(;B[ba];W[ab]C[dead])))