module type KEY = sig

    type 'a t

    val comp: 'a t -> 'b t -> ('a, 'b) Tools.cmp

    val repr: Format.formatter -> 'a t -> unit

end

module Make (El : KEY) : sig

  type t

  (** Create an empty tree *)
  val empty: t

  (** Return the element in the tree with the given key *)
  val find: 'a El.t -> t -> 'a

  (** Add one element in the tree, if the element is already present, it is replaced. *)
  val add: 'a El.t -> 'a -> t -> t

  (** Check if the key exists *)
  val member: 'a El.t -> t -> bool

  val remove: 'a El.t -> t -> t

  (** This type is used in the fold function as existencial type *)
  type container = C : ('a El.t * 'a) -> container [@@unboxed]

  val fold: ('a -> container -> 'a) -> 'a -> t -> 'a

  (** Represent the content in dot syntax *)
  val repr: Format.formatter -> t -> unit

end