Map.Make

`functor (Ord : OrderedType) ->
sig
type key = Ord.t
type +'a t
val empty : 'a t
val is_empty : 'a t -> bool
val add : key -> 'a -> 'a t -> 'a t
val find : key -> 'a t -> 'a
val remove : key -> 'a t -> 'a t
val mem : key -> 'a t -> bool
val iter : (key -> 'a -> unit) -> 'a t -> unit
val map : ('a -> 'b) -> 'a t -> 'b t
val mapi : (key -> 'a -> 'b) -> 'a t -> 'b t
val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int
val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool
end`