def lock_option(self, key, subkey):
key, subkey = _lower_keys(key, subkey)<EOL>_entry_must_exist(self.gc, key, subkey)<EOL>self.gc.loc[<EOL>(self.gc["<STR_LIT>"] == key) &<EOL>(self.gc["<STR_LIT>"] == subkey), "<STR_LIT>"] = True<EOL>
Make an option unmutable.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.

        :raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                option.