at to recapture.
Trying to secure the eye on the right directly with or 'a' doesn't work; white can just destroy the eye on the left.
Black plays at or 'a' is met by . Next white have 'x' to destroy the potential eye on the right and 'y' to destroy the potential eye on the left like the sequence in the previous diagram.
Before or after / exhange, is probably the strongiest resistance, but there's only a gote eye on the left. After , black is still stuck with the miai of 'x' and 'y'.