sig
type t
and 'a tag
val empty : unit -> PropList.t
val make_new_tag : 'a -> 'a PropList.tag
val put : 'a PropList.tag -> 'a -> PropList.t -> unit
val get : 'a PropList.tag -> PropList.t -> 'a
val mem : 'a PropList.tag -> PropList.t -> bool
end