[plt-scheme] verifying whether a symbol is defined at compile time?

Hi all -

Is there a way to verify whether a symbol is defined (or imported) at
compile time?  I am trying to write a conditional define macro (e.g.
define-once) that will evaluate to define if the variable is not previously
defined, but struggling to figure out how to determine whether a symbol's
existence.  (namespace-defined? v) doesn't appear to work for compile-time.


Thanks,
yinso

-- 
http://www.yinsochen.com
...continuous learning...
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