`val find : f:('a -> bool) -> 'a list -> 'a option`
`val rfind : f:('a -> bool) -> 'a list -> 'a option`
`val findi : f:(int -> 'a -> bool) -> 'a list -> (int * 'a) option`
`val split_at : int -> 'a list -> [ `Ok of 'a list * 'a list | `Invalid_argument of string ]`
`val at : 'a list -> int -> [ `Ok of 'a | `Invalid_argument of string ]`
`val assoc : 'a -> ('a * 'b) list -> 'b option`
`val assoc_inv : 'b -> ('a * 'b) list -> 'a option`
`val assq : 'a -> ('a * 'b) list -> 'b option`