def _get_all_children(self,):
        """ 
        return the list of children of a node
        """
        res = ''
        if self.child_nodes:
            for c in self.child_nodes:
                res += ' child = ' + str(c) + '\n'
                if c.child_nodes:
                    for grandchild in c.child_nodes:
                        res += '   child = ' + str(grandchild) + '\n'
        else:
            res += ' child = None\n'
        return res