code: def _get_valid_reff(self, urn, level):
          
          urn = URN(urn)
          subreference = None
          textId=urn.upTo(URN.NO_PASSAGE)
          if urn.reference is not None:
              subreference = str(urn.reference)
          reffs = self.resolver.getReffs(textId=textId, subreference=subreference, level=level)
          r = render_template(
              "cts/GetValidReff.xml",
              reffs=reffs,
              urn=textId,
              level=level,
              request_urn=str(urn)
          )
          return r, 200, {"content-type": "application/xml"}
docs: Provisional route for GetValidReff request
  
          :param urn: URN to filter the resource
          :return: GetValidReff response