type ('a, 'b) t
val create : unit -> ('a, 'b) t
val mem : ('a, 'b) t -> 'a list -> bool
val find : ('a, 'b) t -> 'a list -> 'b
val set : ('a, 'b) t -> 'a list -> 'b -> ('a, 'b) t
val unset : ('a, 'b) t -> 'a list -> ('a, 'b) t
val iter : ('a -> 'b option -> unit) -> ('a, 'b) t -> unit
val iter_path : ('a -> 'b option -> unit) -> ('a, 'b) t -> 'a list -> unit
val fold : ('a -> 'b option -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c
val map : ('b -> 'c option) -> ('a, 'b) t -> ('a, 'c) t
val sub : ('a, 'b) t -> 'a list -> ('a, 'b) t