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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def remove_perm(self, subj_str, perm_str):
 """Remove permission from a subject. Args: subj_str : str Subject for which to remove permission(s) perm_str : str Permission to remove. Implicitly removes all higher permissions. E.g., ``write`` will also remove ``changePermission`` if previously granted. """

        self._assert_valid_permission(perm_str)
        for perm_str in self._equal_or_higher_perm(perm_str):
            self._perm_dict.setdefault(perm_str, set()).discard(subj_str)