type +'a t = private int
val nil : 'a t
val null : 'a t
val is_nil : 'a t -> bool
val print : Format.formatter -> 'a t -> unit
val compare : 'a t -> 'a t -> int
val equal : 'a t -> 'a t -> bool
val hash : 'a t -> int

external to_int : 'a t -> int = "%identity"
external of_int : int -> 'a t = "%identity"