docs: def getValidReff(self, level=1, reference=None):
          """ Given a resource, CtsText will compute valid reffs
  
          :param level: Depth required. If not set, should retrieve first encountered level (1 based)
          :type level: Int
          :param reference: CapitainsCtsPassage reference
          :type reference: CtsReference
          :rtype: list(str)
          :returns: List of levels
          """
          if reference:
              urn = "{0}:{1}".format(self.urn, reference)
          else:
              urn = str(self.urn)
  
          if level == -1:
              level = len(self.citation)
  
          xml = self.retriever.getValidReff(
              level=level,
              urn=urn
          )
          xml = xmlparser(xml)
          self._parse_request(xml.xpath("//ti:request", namespaces=XPATH_NAMESPACES)[0])
  
          return [ref.split(":")[-1] for ref in xml.xpath("//ti:reply//ti:urn/text()", namespaces=XPATH_NAMESPACES)]