-- http://www.mail-archive.com/haskell-cafe@haskell.org/msg10223.html
instance Arbitrary Word64 where
    arbitrary = do 
      let mx,mn :: Integer
          mx = fromIntegral (maxBound :: Word64)
          mn = fromIntegral (minBound :: Word64)
      c <- choose (mx, mn)
      return (fromIntegral c)