docs: def add(self, entity):
          """
          Adds the given entity to this cache.
  
          :param entity: Entity to add.
          :type entity: Object implementing :class:`everest.interfaces.IEntity`.
          :raises ValueError: If the ID of the entity to add is ``None``
            (unless the `allow_none_id` constructor argument was set).
          """
          do_append = self.__check_new(entity)
          if do_append:
              self.__entities.append(entity)