docs: 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)