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

def from_etree(cls, etree_element):
        """
        create a ``DominanceRelation`` instance from an etree element
        representing an <edges> element with xsi:type
        'sDocumentStructure:SDominanceRelation'.
        """
        ins = SaltEdge.from_etree(etree_element)
        # TODO: this looks dangerous, ask Stackoverflow about it!
        # convert SaltEdge into DominanceRelation
        ins.__class__ = DominanceRelation.mro()[0]
        ins.features = get_annotations(etree_element)
        return ins