_get_list_keys

        Low-level api: Given a schema node, in particular, a list type schema
        node, it returns a list of keys.

        Parameters
        ----------

        schema_node : `Element`
            A schema node.

        Returns
        -------

        list
            A list of tags of keys in `{url}tagname` notation.

def _get_list_keys(schema_node):
        '''_get_list_keys

        Low-level api: Given a schema node, in particular, a list type schema
        node, it returns a list of keys.

        Parameters
        ----------

        schema_node : `Element`
            A schema node.

        Returns
        -------

        list
            A list of tags of keys in `{url}tagname` notation.
        '''

        nodes = list(filter(lambda x: x.get('is_key'),
                            schema_node.getchildren()))
        return [n.tag for n in nodes]