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)
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``.