[Haskell-cafe] an instance in other than the last type parameters

Hi, I have probably a very simple question, but I wasn't able to figure it out myself. Consider a two-parameter data type:

data X a b = X a b

If I want to make it a functor in the last type variable (b), I can just define

instance Functor (X a) where fmap f (X a b) = X a (f b)

But how do I write it if I want X to be a functor in its first type variable? Is that possible at all? Something like:

instance Functor ??? where fmap f (X a b) = X (f a) b

Thanks in advance, Petr