docs: def master_ref(self):
          """
          Filters the current DataFrame references to only contain those rows whose reference is master.
  
          >>> master_df = repos_df.master_ref
  
          :rtype: ReferencesDataFrame
          """
          return ReferencesDataFrame(self._engine_dataframe.getReferences().getHEAD(),
                                     self._session, self._implicits)