kb: AFAIK there is no discussion on Sensei's about this invasion - I will create a page with the joseki on it later. However, there's a lot of trick moves played at the amateur level here, namely this - I don't find it in many pro games. Let's use this BQM page to open up discussion about it. (The White stone on the right could also be at a - this is relevant in some continuations.)
Anonymous: This is a fighting move, but right here it's too easy for white to settle. Usually players play one point to the left of to keep their stones connected and their shape strong. For reference I will give the usual variation.
Anonymous: Now black can continue with either 'a' or 'b'.
Anonymous: Right, the lower one is almost the same as 'a'. The upper one looks interesting. Maybe there's a slight difference: the upper-left result is now of the 3-3 joseki. This type of invasion also occurs in the kobayashi fuseki. Anyway, just by playing natural moves you can never go wrong in joseki.
Bill: Ah, but what is natural? ;)
Anonymous: But for your question, the first move that comes to mind is this . Now white will find no troubles settling his shape and black will be relatively thin and weak on the outside. I guess playing 'a' first might be feasible on some situations. Even 'b' if white wants to force some variation. In fact, after black's move, one could consider tenuki'ing because the exchange of - already looks a bit damaging to my eyes. (How can black add another move here that would remove all the aji?) Of course, we would need to see the whole board to find out the correct tactic.
Btw. In this situation, why does black feel the need to attack ? Black can just play at .