Find CRefPattern in the text and set object.citation
        :param xml: Xml Resource
        :type xml: lxml.etree._Element
        :return: None

def _findCRefPattern(self, xml):
        """ Find CRefPattern in the text and set object.citation
        :param xml: Xml Resource
        :type xml: lxml.etree._Element
        :return: None
        """
        if not self.citation.is_set():
            citation = xml.xpath("//tei:refsDecl[@n='CTS']", namespaces=XPATH_NAMESPACES)
            if len(citation):
                self.citation = Citation.ingest(resource=citation[0], xpath=".//tei:cRefPattern")
            else:
                raise MissingRefsDecl("No reference declaration (refsDecl) found.")