docs: def get_highest_perm_str(self, subj_str):
          """
      Args:
        subj_str : str
          Subject for which to retrieve the highest permission.
  
      Return:
         The highest permission for subject or None if subject does not have any permissions.
      """
          pres_perm_set = self._present_perm_set_for_subj(self._perm_dict, subj_str)
          return (
              None if not pres_perm_set else self._highest_perm_from_iter(pres_perm_set)
          )