Registers the given entity for the given class as CLEAN.

        :returns: Cloned entity.

def register_clean(self, entity_class, entity):
        """
        Registers the given entity for the given class as CLEAN.

        :returns: Cloned entity.
        """
        EntityState.manage(entity, self)
        EntityState.get_state(entity).status = ENTITY_STATUS.CLEAN
        self.__entity_set_map[entity_class].add(entity)