Alex: There are many variations, but unless I've read wrong, Black trying either or at a fails, so his best attempt at this. Now, at a either lets Black live in the corner or gets White into a capturing race she can't win, so...
Alex: Black loses the capturing race. Of course, this assumes that the Black stones on the outside are already alive, because Black can play at and connect through that way. If Black invaded before the outside was safe, however, White would let him live in the corner in gote and strengthen herself outside to continue attacking.
Still, although White keeps the corner, this is bad aji, because at will still cut off the outside stones. White must make sure they get strong enough during the attack that they can take care of themselves.
tderz: Black offers more resistance to White's clever counter
tderz: at is not good for White
=a (better not at b)
I forgot the correct terminology for this type of ko.
White has 2 internal ko threats at c and e.
However, Black could also chose to neglect the 1st one (c) and take at b...
Bill: Because of the weakness, I was thinking about Wa or even Wb. After seeing this page, Wb starts looking better.