If black captures with , white starts a ko in the corner to capture three stones. Black cannot connect because of oiotoshi.
If black plays to prevent the ko,
results in double damezumari, and thus captures the
stones.
Preventing the ko with doesn't work either. White throws in with
, and after
it is double damezumari again. This throw-in also work if
is at a.
This , preventing the throw-in, results in ko again. Although defending with
at
is no longer oiotoshi, white will then play
at a and it is double damezumari yet again.