David: My attempt:
If White plays 2 at a, Black simply connects at 3.
The throw-in at 2 here is fruitless for White.
I think this works as well, but I'm not sure:
Black 1 creates a bent four eyespace. It seems to me that this doesn't
lead to a ko as in Bent Four in the Corner is Dead, because neither of
the central points of the bent four are in the corner.
Tsumego from games 23