docs: def _highest_perm_dict_from_perm_dict(self, perm_dict):
          """Return a perm_dict where only the highest permission for each subject is
          included."""
          highest_perm_dict = copy.copy(perm_dict)
          for ordered_str in reversed(ORDERED_PERM_LIST):
              for lower_perm in self._lower_perm_list(ordered_str):
                  highest_perm_dict.setdefault(lower_perm, set())
                  highest_perm_dict[lower_perm] -= perm_dict.get(ordered_str, set())
          return highest_perm_dict