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