class  (Eq a) => Ord a  where
   compare              :: a -> a -> Ordering
    (<), (<=), (>=), (>) :: a -> a -> Bool
    max, min             :: a -> a -> a

        -- Minimal complete definition:
        --      (<=) or compare
        -- Using compare can be more efficient for complex types.
    compare x y
         | x == y    =  EQ
         | x <= y    =  LT
         | otherwise =  GT

    x <= y           =  compare x y /= GT
    x <  y           =  compare x y == LT
    x >= y           =  compare x y /= LT
    x >  y           =  compare x y == GT

-- note that (min x y, max x y) = (x,y) or (y,x)
    max x y 
         | x >= y    =  x
         | otherwise =  y
    min x y
         | x <  y    =  x
         | otherwise =  y