[plt-scheme] renaming primitives

Hello all,

I've run into a slight problem:

(module x mzscheme
   (set! gensym (lambda () '123))

.... use gensym later...
)

I want to make gensym do something else, so I thought it would be fine
to set! it to some new function, but it's built into mzscheme and I
can't mutate module-required variables.  I can't use (require (rename
mzscheme old-gensym gensym)) because I already need to require mzscheme
as the initial-module.
How do I redefine gensym in a module?

Regards,

-----------------------------------
Daniel Silva
dsilva at ccs.neu.edu