xela: I think this does the trick.
xela: is a mistake. After and , black can live with either 'a' or 'b'.
unkx80: I have not seen the solution yet, but here's some food for thought. What if is sente in the sense that it threatens to escape to the side? Then if White avoids ko, Black can live. (If at , then at .)
How about at ?
can play at . This should solve the sente probem and keep it dead.
jms: If at (from the previous diagram), the result is Seki - black can make Seki at any time by playing at A. If Black mistakenly plays at B, however, Black dies.