new_contents: from bento.commands.cmd_contexts \
      import \
          _RegistryBase
  from bento.compat.api import moves
  
  class Test_RegistryBase(moves.unittest.TestCase):
      def test_simple(self):
          registry = _RegistryBase()
          registry.register_category("dummy", lambda: 1)
          registry.register_callback("dummy", "dummy_func", lambda: 2)
  
          self.assertEqual(registry.callback("dummy", "dummy_func")(), 2)
          self.assertEqual(registry.callback("dummy", "non_existing_dummy_func")(), 1)
  
      def test_double_registration(self):
          registry = _RegistryBase()
          registry.register_category("dummy", lambda: 1)
          self.assertRaises(ValueError, lambda: registry.register_category("dummy", lambda: 2))
  
          self.assertEqual(registry.callback("dummy", "non_existing_dummy_func")(), 1)
  
      def test_missing_category(self):
          registry = _RegistryBase()
          self.assertRaises(ValueError, lambda: registry.register_callback("dummy", "dummy_func", lambda: 2))
          self.assertRaises(ValueError, lambda: registry.callback("dummy", "dummy_func"))
  
      def test_default_callback(self):
          registry = _RegistryBase()
  
          registry.register_category("dummy", lambda: 1)
          self.assertEqual(registry.default_callback("dummy"), 1)
          self.assertRaises(ValueError, lambda: registry.default_callback("non_existing_category"))
  
subject: add some tests for _RegistrBase class.