[racket] Check for #<undefined>

From: Todd O'Bryan (toddobryan at gmail.com)
Date: Tue May 31 18:29:22 EDT 2011

I have a structure with a guard function. It works well.

However, I want to use it in a (shared ...) construct, and when the
guard runs, a value is #<undefined>. I'm fine with letting the guard
accept #<undefined> as a legal value, but I can't figure out any way
to test for it.

Is there such a beast, or is #<undefined> untouchable?


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