once we have ordNub, we can create lots of RULES like the following

{-# RULES "nub/ordNub" nub = ordNub :: [String] -> [String] #-} 

{-# RULES "nub/ordNub" nub = ordNub :: [Int] -> [Int] #-} 

{-# RULES "nub/ordNub" nub = ordNub :: [Integer] -> [Integer] #-} 

actually, we probably want one that uses IntMap for ints. it is

signifigantly faster.

I don't suppose there is a way to match a RULE to a class constraint? I

don't see how it would be implemented in any easy way since classes get

desugared away pretty early... but it would be an interesting feature..

{-# RULES "nub/ordNub" forall (a::*) . Ord a => nub = ordNub :: [a] -> [a] #-} 

or something...