prop_unionWithSelfIsSelf :: SetOf e s => s -> Bool
prop_unionWithSelfIsSelf s = (s `union` s) `sameAs` s
prop_unionWithSelfIsSelf :: SetOf e s => s -> Bool
prop_unionWithSelfIsSelf s = (s `union` s) `sameAs` s
prop_unionWithSelfIsSelf s = (s `union` s) `sameAs` s