code: def set_mrt(self, s, mrt: str):
          
          valid_mrts_abv = self.ABV_MRT.keys()
          valid_mrts_uri = self.ABV_MRT.values()
          if mrt in valid_mrts_abv:
              mrt_uri = self.ABV_MRT[mrt]
          elif mrt in valid_mrts_uri:
              mrt_uri = mrt
          else:
              raise ValueError("mrt must be one of {}, found {}".format(valid_mrts_abv, mrt))
          mrt_qid = self.mrt_qids[mrt_uri]
  
          q = wdi_core.WDItemID(mrt_qid, self.mrt_pid, is_qualifier=True)
          s.qualifiers.append(q)
          return s
docs: accepts a statement and adds a qualifer setting the mrt
          modifies s in place
          :param s: a WDBaseDataType statement
          :param mrt: one of {'close', 'broad', 'exact', 'related', 'narrow'}
          :return: s