method add_item (a:string) (b:string) (c:int8) l =
(* add a new item to the list stats *)
if (List.length l) = 0 then
[(a,b,c)]
else (* there's at least 1 element *)
let premier = (List.hd l) in
let (x,y,z) = premier in
let reste = (List.tl l) in
if (x,y)=(a,b) then
List.append [(x,y,(plus c z))] reste
else
List.append [premier] (self#add_item a b c reste)