docs: def root_node(self, value):
          """
          Setter for **self.__root_node** attribute.
  
          :param value: Attribute value.
          :type value: AbstractCompositeNode
          """
  
          if value is not None:
              assert issubclass(value.__class__, AbstractCompositeNode), \
                  "'{0}' attribute: '{1}' is not a '{2}' subclass!".format(
                      "root_node", value, AbstractCompositeNode.__name__)
          self.__root_node = value