val find : 'a t -> int -> int -> int -> int -> int -> int -> 'a
val add : 'a t -> int -> int -> int -> int -> int -> int -> 'a -> unit
val dummy : 'a t -> 'a
val find : 'a t -> int -> int -> int -> int -> int -> int -> 'a
val add : 'a t -> int -> int -> int -> int -> int -> int -> 'a -> unit
val dummy : 'a t -> 'a