[plt-scheme] Contracts: having ->r and opt->

Hello all,

Is it possible to have contracts for opt-lambda which arguments depend
on each other?

For example:
(define foo
   (opt-lambda (x y [z 0])
    ....))

x, y and z should be numbers and y should be bigger than x. Is it
possible to say this in a contract?

Cheers,

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