[ODE] subtractive geoms?

[Fig TaTam]

How hard would it be to add "subtractive" geoms to ODE? For example, you 
could have a large sphere and add a smaller subtractive sphere inside it, so 
objects could bounce inside the sphere. Or you could have a capped cylinder 
and subtract a sphere from one of it's caps. So the result is a cylinder 
with one cap pointing inside the cylinder. You could do all sorts of complex 
geoms with that kind of thing.
Would it be too computationally expensive to do?

_________________________________________________________________
MSN 8 with e-mail virus protection service: 2 months FREE*  
http://join.msn.com/?page=features/virus