[plt-scheme] determine free variables

Hi all -

What's the best approach to determine free variables at source level?

What i am trying to accomplish is to take a scheme expression from a file,
say

(+ foo bar)

and transform

(lambda (foo bar)
  (+ foo bar))

for arbitrary scheme expressions.

Are there procedures that would allow me to determine whether a symbol has a
binding?  Given scheme identifier can be bound to different values, it seems
like this would require some sort of runtime environment inspection - is
that true?

Thanks,
yc
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