creates an etree element of a ``SaltNode`` that mimicks a SaltXMI
        <nodes> element

def to_etree(self):
        """
        creates an etree element of a ``SaltNode`` that mimicks a SaltXMI
        <nodes> element
        """
        layers_attrib_val = ' '.join('//@layers.{}'.format(layer_id)
                                     for layer_id in self.layers)

        attribs = {
            '{{{pre}}}type'.format(pre=NAMESPACES['xsi']): self.xsi_type,
            'layers': layers_attrib_val}

        E = ElementMaker()
        node = E('nodes', attribs)
        label_elements = (label.to_etree() for label in self.labels)
        node.extend(label_elements)
        return node