--- /dev/null
+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"