create a ``TextualRelation`` instance from an etree element
        representing an <edges> element with xsi:type
        'sDocumentStructure:STextualRelation'.

def from_etree(cls, etree_element):
        """
        create a ``TextualRelation`` instance from an etree element
        representing an <edges> element with xsi:type
        'sDocumentStructure:STextualRelation'.
        """
        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