[Haskell-beginners] defining own arbitrary :: Gen Int

[ben]

Hello,

I would like to define my own arbitrary which should only give naturals,
i. e. which should behave like

instance Arbitrary Integer where
        arbitrary = elements ([0..]).

This definition gives a "multiple definition" error. Is there a way to
hide the definition of arbitrary::Gen Int of QuickCheck, such that I can
use my own definition ?

Thanks a lot in advance,
ben