panic using type families and type classes

I got a panic while trying to compile some code using type families:

ghc-6.9.20080528: panic! (the 'impossible' happened)
  (GHC version 6.9.20080528 for i386-apple-darwin):
        urk! lookup local fingerprint

I can't post the code that caused that (yet) but when I was trying to distill it down to its essence for a bug report, I got a different panic:

ghc-6.9.20080528: panic! (the 'impossible' happened)
  (GHC version 6.9.20080528 for i386-apple-darwin):
        Prelude.head: empty list

I've attached the code that caused it. I know the code might not be 100% correct, and it's a bit long, but I think it might help some.

Edited Mar 09, 2019 by Simon Marlow

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