let previous_add_stack (() : unit) : unit = match (!add_stack_story) with [] -> ()
|(x::ls) -> (List.iter
(fun (name_in_session,message) -> (
let message_list = Hashtbl.find_all message_stack name_in_session
in
let rec aux l buff =
match l with [] -> (List.iter (fun t -> (Hashtbl.add message_stack name_in_session t)) buff)
|(x::ls) -> ( Hashtbl.remove message_stack name_in_session;
if (x=message)
then(aux ls buff)
else(aux ls (x::buff)));
in
aux message_list [];
)) (!add_on_stack);
add_on_stack:=x;
add_stack_story:=ls)