<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_perm(self, subj_str, perm_str):
 """Add a permission for a subject. Args: subj_str : str Subject for which to add permission(s) perm_str : str Permission to add. Implicitly adds all lower permissions. E.g., ``write`` will also add ``read``. """

        self._assert_valid_permission(perm_str)
        self._perm_dict.setdefault(perm_str, set()).add(subj_str)