module MakeGap (E : DefaultValType) : OfArrayGap(MakeArray(E))(E)
Vectors with gaps from usual arrays. get v i and set v i e are allowed
    for all i >= 0.
type E.t 
val default : unit -> E.t
type 
|    | mutable v : A.t; | 
|    | mutable n : int; | 
}
val make : ?size:int -> unit -> t
val set : t -> int -> A.elt -> unit
val get : t -> int -> E.t
val clear : t -> unit
val length : t -> int
val capacity : t -> int
val index_max : t -> int
val blit : t -> int -> t -> int -> int -> unit
val default : unit -> E.t