Set complement

Hi,

I couldn't find a set complement filter so I ended up putting one together, did I miss it in Jedi?

I've attached a patch in case you're interested or had any feedback (it's been a while since I did much with set theory).

I went for "minus" as a name, so the set complement of A in B (B \ A) would look like;

select(B, minus(A));

I think I would have preferred select(A, in(B)) but hey...

Cheers,
Toby