ghc 7 produces these warnings http://hackage.haskell.org/trac/ghc/ticket/4288
The specialization is enabled by -O2, and the default limit of 3 is
there to avoid specialization blowing up binary size. Perhaps that default
is a little low? I needed 4 to avoid a warning on Unused.hs, and 5 to avoid
warnings on test.hs