`val find : f:('a -> bool) -> 'a t -> 'a option`
`val rfind : f:('a -> bool) -> 'a t -> 'a option`
`val findi : f:(int -> 'a -> bool) -> 'a t -> (int * 'a) option`
`val rfindi : f:(int -> 'a -> bool) -> 'a t -> (int * 'a) option`
`val split_at : int -> 'a t -> [ `Ok of 'a t * 'a t | `Invalid_index of int ]`
`val at : 'a t -> int -> [ `Ok of 'a | `Invalid_index of int ]`
`val assoc : 'a -> ('a * 'b) t -> 'b option`
`val assq : 'a -> ('a * 'b) t -> 'b option`