type waiter = {
  promise : string option Eio.Promise.t;
  resolver : string option Eio.Promise.u;
}

type t

val create : unit -> t
val register : t -> seq:int -> waiter
val complete : t -> seq:int -> payload:string option -> bool

val wait :
  waiter -> timeout:float -> clock:_ Eio.Time.clock -> string option option

val cancel : t -> seq:int -> unit
val pending_count : t -> int