Return the count of references in this part's XML to the relationship
        identified by *rId*.

def _rel_ref_count(self, rId):
        """
        Return the count of references in this part's XML to the relationship
        identified by *rId*.
        """
        rIds = self._element.xpath('//@r:id')
        return len([_rId for _rId in rIds if _rId == rId])