[racket] inspect variable values in backtrace?

Hi all,

Is it possible to see the bindings of various things in a backtrace?  I am
having trouble debugging my code because I'm not sure which input is causing
it to fail -- at some point, I'm trying to take the car of an empty list.
 The outer thing is a for-each mapping over a hash table with about 500
entries, and I'm not sure which entry is causing my inner stuff to break.

Can't use printf, right?... any advice?

thanks,
Fred
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