code: def from_etree(cls, etree_element):
          
          ins = SaltEdge.from_etree(etree_element)
          # TODO: this looks dangerous, ask Stackoverflow about it!
          # convert SaltEdge into TextualRelation
          ins.__class__ = TextualRelation.mro()[0]
          ins.onset = get_string_onset(etree_element)
          ins.offset = get_string_offset(etree_element)
          return ins
docs: create a ``TextualRelation`` instance from an etree element
          representing an <edges> element with xsi:type
          'sDocumentStructure:STextualRelation'.