method get_id_of role_id string_id =
try
Hashtbl.find to_id (role_id,string_id)
with
Not_found ->
(let var_num = var_number in
Hashtbl.add to_id (role_id,string_id) var_num ;
Hashtbl.add from_id var_num (role_id,string_id);
var_number <- var_number+1 ;
var_num)