docs: def _write_subset_index_file(options, core_results):
      """
      Write table giving index of subsets, giving number and subset string
      """
  
      f_path = os.path.join(options['run_dir'], '_subset_index.csv')
      subset_strs = zip(*core_results)[0]
      index = np.arange(len(subset_strs)) + 1
      df = pd.DataFrame({'subsets': subset_strs}, index=index)
      df.to_csv(f_path)