docs: def _flatten_plus_safe(tmp_dir, rollback_files):
      """Flatten names of files and create temporary file names.
      """
      tx_fpaths, orig_files = [], []
      for fnames in rollback_files:
          if isinstance(fnames, six.string_types):
              fnames = [fnames]
          for fname in fnames:
              tx_file = fname + '.tx'
              tx_fpath = join(tmp_dir, tx_file) if tmp_dir else tx_file
              tx_fpaths.append(tx_fpath)
              orig_files.append(fname)
      return tx_fpaths, orig_files