code: def from_etree(cls, etree_element):
          
          ins = SaltElement.from_etree(etree_element)
          # TODO: this looks dangerous, ask Stackoverflow about it!
          # convert SaltElement into SaltLayer
          ins.__class__ = SaltLayer.mro()[0]
  
          # add nodes and edges that belong to this layer (if any)
          for element in ('nodes', 'edges'):
              elem_list = []
              xpath_result = etree_element.xpath('@'+element)
              if xpath_result:
                  val_str = xpath_result[0]
                  elem_list.extend(int(elem_id)
                                   for elem_id in DIGITS.findall(val_str))
              setattr(ins, element, elem_list)
          return ins
docs: creates a ``SaltLayer`` instance from the etree representation of an
          <layers> element from a SaltXMI file.