type t

val create : ?init:bool -> int -> t
val alloc :  int -> t
val length : t -> int
val set : t -> int -> bool -> unit
val get : t -> int -> bool

val unsafe_set : t -> int -> bool -> unit
val unsafe_get : t -> int -> bool
val union : t -> t -> t
val inter : t -> t -> t
val diff  : t -> t -> t