def testAdditionalCategories(self):
   track = TracingTrack(None, additional_categories=('best-category-ever',))
   self.assertIn('best-category-ever', track.Categories())
   # Cannot re-enable a category.
   with self.assertRaises(AssertionError):
     TracingTrack(None, additional_categories=('cc',))
   # Cannot disable categories.
   with self.assertRaises(AssertionError):
     TracingTrack(None, additional_categories=('-best-category-ever',))