let bigint_times list1 list2 =
  let rec f list1_rec list2_rec puissance =
  match (list1_rec,list2_rec) with
    ([],[])->[]
    | ([],_)|(_,[])->[]
    | (e1::l1,l2)->
        bigint_sum(bigint_mult l2 (e1*puissance))(f l1 l2 (puissance*10))
in f list1 list2 1;;