code: def to_etree(self):
          
          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
docs: creates an etree element of a ``SaltNode`` that mimicks a SaltXMI
          <nodes> element