code: def _get_first_urn(self, urn):
          
          urn = URN(urn)
          subreference = None
          textId = urn.upTo(URN.NO_PASSAGE)
          if urn.reference is not None:
              subreference = str(urn.reference)
          firstId = self.resolver.getTextualNode(textId=textId, subreference=subreference).firstId
          r = render_template(
              "cts/GetFirstUrn.xml",
              firstId=firstId,
              full_urn=textId,
              request_urn=str(urn)
          )
          return r, 200, {"content-type": "application/xml"}
docs: Provisional route for GetFirstUrn request
  
          :param urn: URN to filter the resource
          :param inv: Inventory Identifier
          :return: GetFirstUrn response