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

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