let get_group_previous_project gnum padd =
let rec find_rec last l = match l with
| [] -> raise Not_found
| x::c -> if (x = padd) then last
else find_rec x c in
let proj = get_group_projects gnum in
let nums = List.sort compare (List.map (get_paddress) proj) in
find_rec (List.hd nums) (List.tl nums)