let previous_remove_stack (() : unit) :unit = match (!remove_stack_story) with [] -> ()
                                                         |(x::ls) ->         (List.iter
                                                                        (fun (name_in_session,message) -> Hashtbl.add message_stack name_in_session message)
                                                                        (!remove_of_stack);
                                                                        remove_of_stack:=x;
                                                                        remove_stack_story:=ls)