docs: 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