@Override
 public FixtureProvider[] fixtureProviders() {
   return new FixtureProvider[] {
     new Db4oFixtureProvider(),
     new SimpleFixtureProvider(
         SET_SPEC,
         new CollectionSpec<HashSet<CollectionElement>>(
             HashSet.class,
             CollectionFactories.activatableHashSetFactory(),
             CollectionFactories.plainHashSetFactory()),
         new CollectionSpec<TreeSet<CollectionElement>>(
             TreeSet.class,
             CollectionFactories.activatableTreeSetFactory(),
             CollectionFactories.plainTreeSetFactory())),
   };
 }