Solution by lavalyn:
eliminates the eye at the edge - a and b are miai for the white group escape to safety.
While the peep of must be answered, the kill still requires .
Curious to know what would happen if was played. Looks right in my head.
EthrDemon?: Doesn't the miai of the first solution also give B a ko threat?
Dudzik: Yes, but this way gives black two of them: black a forces white b, and black still has the same threat as in the first solution.