docs: def requestClaimDef(self, seqNo, signature, sender):
          """
          Used to get a claim def from Sovrin
          :param seqNo: reference number of schema
          :param signature: CL is only supported option currently
          :return: req object
          """
          operation = { TXN_TYPE: GET_CLAIM_DEF,
                        ORIGIN: sender,
                        REF : seqNo,
                        SIGNATURE_TYPE : signature
                      }
  
          req = Request(sender, operation=operation)
          return self.prepReq(req)