Attribute referencing the tree ancestors of the node instance.

        :returns: list of node ancestors in sequence, first item is 
            the current node instance (`self`), the last item is root.
        :rtype: list of Node references

def _ancestors(self):
        """Attribute referencing the tree ancestors of the node instance.

        :returns: list of node ancestors in sequence, first item is 
            the current node instance (`self`), the last item is root.
        :rtype: list of Node references
        """
        # return list of ancestor nodes starting with self.parent and ending with root
        _ancestors=[]
        _n = self
        while _n.parent:
            _n = _n.parent
            _ancestors.append(_n)
        return _ancestors