Get all valid reffs for every part of the CtsText

        :rtype: [str]

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

        :rtype: [str]
        """
        if not self._cts:
            self._cts = BaseReferenceSet(
                [reff for reffs in [self.getReffs(level=i) for i in range(1, len(self.citation) + 1)] for reff in reffs]
            )
        return self._cts