instance declaration troubles

[Nick Name]

Yes, I usually RTFM before posting, but you have misunderstood my
question (however, thanks for always reading and answering newbie
questions like mine); what I want to do is the 

instance (Get a) => Functor a where
     fmap f x = mk (ls x >>= return . map f)

Now, what I mean is: "any type in Get class is also in Functor class,
and I tell you how". But I need undecidable instances! Why? Is there a
simple way to state this property, that the Get class is a subset of the
Functor class?

Vincenzo