# Module type Fold_map.S2

    module type S2 = sig .. end

    type 'a out_value

    type ('key, 'data) t = private ('key, 'data out_value) Core.Std.Map.Poly.t

    type 'a _in_value = 'a

    type 'a _out_value = 'a out_value

    type ('a, 'b) _t = ('a, 'b) t

    val empty : ('a, 'b) t

    val singleton : 'a -> 'b -> ('a, 'b) t

    val is_empty : ('a, 'b) t -> bool

    val length : ('a, 'b) t -> int

    val add : key:'a -> data:'b -> ('a, 'b) t -> ('a, 'b) t

    val find : ('a, 'b) t -> 'a -> 'b out_value

    val remove : ('a, 'b) t -> 'a -> ('a, 'b) t

    val set : key:'a ->
           data:'b out_value ->
           ('a, 'b) t -> ('a, 'b) t

    val mem : ('a, 'b) t -> 'a -> bool

    val iter : ('a, 'b) t ->
           f:(key:'a -> data:'b out_value -> unit) -> unit

    val fold : ('a, 'b) t ->
           init:'c -> f:(key:'a -> data:'b out_value -> 'c -> 'c) -> 'c

    val filter : ('a, 'b) t ->
           f:(key:'a -> data:'b out_value -> bool) -> ('a, 'b) t

    val keys : ('a, 'b) t -> 'a list

    val data : ('a, 'b) t -> 'b out_value list

    val to_alist : ('a, 'b) t -> ('a * 'b out_value) list

    val of_list : ('a * 'b) list -> ('a, 'b) t

    val for_all : ('a, 'b) t -> f:('b out_value -> bool) -> bool

    val exists : ('a, 'b) t -> f:('b out_value -> bool) -> bool

    val to_map : ('a, 'b) t -> ('a, 'b out_value) Core.Std.Map.Poly.t

    val of_map : ('a, 'b out_value) Core.Std.Map.Poly.t -> ('a, 'b) t