module Plug:Asig..end
plug is something where plug-ins plug.
We build plug on the top of Doc.item.class['a]plug :'b -> ?register_hook:string * 'a -> unit -> string -> bool -> bool ->object..end
plug plugs in plugsval plugs : Doc.itemval create : ?duplicates:bool ->
?register_hook:(string * 'a -> unit) ->
?insensitive:bool -> ?doc:string -> string -> 'a plugval list : unit -> string list