<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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