`____` has an empty middle between the two `__` pairs and does not match `/^__.+__$/`; assert that (not a match) and add `__x__` as the shortest real dunder form that does.
`____` has an empty middle between the two `__` pairs and does not match `/^__.+__$/`; assert that (not a match) and add `__x__` as the shortest real dunder form that does.