OCaml library : Lazy

`sig
type 'a t = 'a CamlinternalLazy.t
exception Undefined
external force : 'a Lazy.t -> 'a = "%lazy_force"
val map : ('a -> 'b) -> 'a Lazy.t -> 'b Lazy.t
val is_val : 'a Lazy.t -> bool
val from_val : 'a -> 'a Lazy.t
val map_val : ('a -> 'b) -> 'a Lazy.t -> 'b Lazy.t
val from_fun : (unit -> 'a) -> 'a Lazy.t
val force_val : 'a Lazy.t -> 'a
val lazy_from_fun : (unit -> 'a) -> 'a Lazy.t
val lazy_from_val : 'a -> 'a Lazy.t
val lazy_is_val : 'a Lazy.t -> bool
end`