Clarify relationship between Functor and Applicative

[Ross Paterson]

On Fri, Feb 25, 2011 at 05:34:18PM -0500, roconnor at theorem.ca wrote:
> In the applicative documentation, it says for an Applicative functor f:
> 
> The Functor instance should satisfy
> 
>       fmap f x = pure f <*> x
> 
> I think the documentation should be clarified that this does not
> need to be checked because it is a consequence of the other
> applicative laws.
> 
> See <http://hpaste.org/44315/applicative_implies_functor>.

Do you have a proof that Functor instances are uniquely determined?