Get all valid reffs for every part of the CtsText

        :rtype: MyCapytain.resources.texts.tei.XmlCtsCitation

def reffs(self):
        """ Get all valid reffs for every part of the CtsText

        :rtype: MyCapytain.resources.texts.tei.XmlCtsCitation
        """
        if not self.citation.is_set():
            self.getLabel()
        return [
            reff for reffs in [self.getValidReff(level=i) for i in range(1, len(self.citation) + 1)] for reff in reffs
        ]