I hate this error. Whilst arguably this is an "untested" extension to Standard Haskell, it seemed pretty sound in practice. Can't we at least have it back as a GlaExt? (MSExt?) I'm starting to feel more than a little Quaint having to use ghc-3.01, just to get my programs to compile... (If anyone has a good systematic workaround to suggest, I'm all ears.) ghc-4.00 -c Intervals.hs -H30m -K2M -recomp -fglasgow-exts -cpp -syslib misc -Rgc-stats -dshow-passes -fmax-simplifier-iterations4 -funfolding-use-threshold-0 -fvia-C Intervals.hs:460: Illegal constaint `Ord (s e)' in instance context (Instance contexts must constrain only type variables) [etc, etc] Slainte, Alex.