code: def is_sibling_of(self, node, subject):
          
          # Ensure nodes are not siblings of themselves.
          return not node.id == subject.id and node.parent_id == subject.parent_id
docs: Returns ``True`` if the given `node` is a sibling of the given
              `subject` node, ``False`` otherwise. This method uses the
              :attr:`parent` field, and so does not perform any query.
  
              :param node: the :class:`CTENode' for which to determine whether it
                  is a sibling of the `subject`.
  
              :param subject: the :class:`CTENode` for which to determine whether
                  one of its siblings is the `node`.
  
              :returns: ``True`` if `node` is a sibling of `subject`, ``False``
                  otherwise.