[plt-scheme] Load unit after macro expansion

Hello all,

I'm trying to define a system which is based on runtime loadable
units. However, these units are covered by a language I defined which
should be converted into a unit. So basically, I have a file with a
(define-type ...) (some macro I defined in my system kernel), which
expands to a a-unit (that's what I thought it would be best at the
time, but it could also expand only to a unit, not sure what's best).
That unit should then be loaded. Problem is, I have to do this at
runtime.

How can I macro-expand the source code on a file and evaluate it during runtime?
Moreover, should I expand to an a-unit or a unit would be enough?

-- 
Paulo Jorge Matos - pocm at soton.ac.uk
http://www.personal.soton.ac.uk/pocm
PhD Student @ ECS
University of Southampton, UK