code: def from_etree(cls, etree_element):
          
          ins = SaltElement.from_etree(etree_element)
          # TODO: this looks dangerous, ask Stackoverflow about it!
          ins.__class__ = SaltEdge.mro()[0]  # convert SaltElement into SaltEdge
          ins.layers = get_layer_ids(etree_element)
          ins.source = get_node_id(etree_element, 'source')
          ins.target = get_node_id(etree_element, 'target')
          return ins
docs: creates a ``SaltEdge`` instance from the etree representation of an
          <edges> element from a SaltXMI file.