creates an etree element of a ``SaltLayer`` that mimicks a SaltXMI
        <layers> element

def to_etree(self):
        """
        creates an etree element of a ``SaltLayer`` that mimicks a SaltXMI
        <layers> element
        """
        nodes_attrib_val = ' '.join('//@nodes.{}'.format(node_id)
                                    for node_id in self.nodes)
        edges_attrib_val = ' '.join('//@edges.{}'.format(edge_id)
                                    for edge_id in self.edges)

        attribs = {
            '{{{pre}}}type'.format(pre=NAMESPACES['xsi']): self.xsi_type,
            'nodes': nodes_attrib_val, 'edges': edges_attrib_val}
        # a layer might have no nodes or edges attributed to it
        non_empty_attribs = {key: val for (key, val) in attribs.items()
                             if val is not None}

        E = ElementMaker()
        layer = E('layers', non_empty_attribs)
        label_elements = (label.to_etree() for label in self.labels)
        layer.extend(label_elements)
        return layer