sig
val get_level : unit -> int
val set_level : int -> unit
val map : ?level:int -> ('a -> 'b) -> 'a list -> 'b list
val iter : ?level:int -> ('a -> unit) -> 'a list -> unit
val fold :
?level:int -> ('a -> 'b -> 'a) -> 'a -> 'b list -> ('a -> 'a -> 'a) -> 'a
end