[racket-dev] futures waiting for scheme_make_envunbox

While trying to use futures to parallelize a simple piece of code, I
was able to remove all of the waiting except for this:

future: 3 waiting for runtime at 1282743524205.783936: [scheme_make_envunbox]

which happens continuously.  What causes this function to be invoked,
and how can I eliminate it?
-- 
sam th
samth at ccs.neu.edu