class  (Eq a, Show a) => Num a  where
    (+), (-), (*)    :: a -> a -> a
    negate           :: a -> a
    abs, signum      :: a -> a
    fromInteger      :: Integer -> a

        -- Minimal complete definition:
        --      All, except negate or (-)
    x - y            =  x + negate y
    negate x         =  0 - x