`val to_int : [> `Read ] t -> int option`
`val to_float : [> `Read ] t -> float option`
`val index : [> `Read ] t -> char -> int option`
`val rindex : [> `Read ] t -> char -> int option`
`val index_from : [> `Read ] t -> int -> char -> int option`
`val rindex_from : [> `Read ] t -> int -> char -> int option`
`val find : [> `Read ] t -> [> `Read ] t -> int option`
`val find_from : [> `Read ] t -> int -> [> `Read ] t -> int option`
`val rfind : [> `Read ] t -> [> `Read ] t -> int option`
`val rfind_from : [> `Read ] t -> int -> [> `Read ] t -> int option`
`val split : [> `Read ] t -> by:[> `Read ] t -> (_ t * _ t) option`
`val rsplit : [> `Read ] t -> by:[> `Read ] t -> (_ t * _ t) option`