Bug in compiling large projects ?

Hello,

I have send a message to these lists a weak ago with almost the same
topic, but I have now additional observations: The problem is not
related to option -O in GHC 6.2.1 ! We use in conjunction these command
line switches for the compilation:
  --make -fglasgow-exts -fallow-overlapping-instances
  -fallow-undecidable-instances

and get this error message, when we try to compile the complete project:

Compiling Common.Lib.Rel   ( ./Common/Lib/Rel.hs, ./Common/Lib/Rel.o )

./Common/Lib/Rel.hs:102:
    Could not unambiguously deduce (Show (a, a))
        from the context (Show (Rel a), Show a, Ord a)
      arising from use of `show' at ./Common/Lib/Rel.hs:102
    The choice of (overlapping) instance declaration
        depends on the instantiation of `a'
    Probable fix:
        Add (Show (a, a)) to the class or instance method `show'
        Or add an instance declaration for (Show (a, a))
    In the first argument of `(.)', namely `show'
    In the definition of `show':
        show = show . (Common.Lib.Set.fromList . toList)
    In the definition for method `show'

When we issue the same ghc call again, Common/Lib/Rel is compiled
without errors. 
Well, the needed tar-ball (4.2M) to reproduce the error is downloadable
at 
http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/CoFI/hets/hets-src.tgz

Cheers,
        Klaus
-- 
Klaus Lüttich    Email:   luettich@xxxxxx
FB 3             Phone:   +49-421-218-7129
Uni Bremen