code: def master_ref(self):
          
          return ReferencesDataFrame(self._engine_dataframe.getMaster(),
                                     self._session, self._implicits)
          return self.ref('refs/heads/master')
docs: Filters the current DataFrame to only contain those rows whose reference is master.
  
          >>> master_df = refs_df.master_ref
  
          :rtype: ReferencesDataFrame