A type for making staging explicit in the type of a function. For example, you might want to have a function that creates a function for allocating unique identifiers. Rather than using the type:
    val make_id_allocator : unit -> unit -> int

you would have

    val make_id_allocator : unit -> (unit -> int) Staged.t

Such a function could be defined as follows:

    let make_id_allocator () =
      let ctr = ref 0 in
      stage (fun () -> incr ctr; !ctr)

and could be invoked as follows:

    let (id1,id2) =
      let alloc = unstage (make_id_allocator ()) in
      (alloc (), alloc ())

both stage and unstage functions are available in Common.

(Note that in many cases, including perhaps the one above, it's preferable to create a custom type rather than use Staged.)

type +'a t
val stage : 'a -> 'a t
val unstage : 'a t -> 'a