(* 3^e in GF(2^8) *)
Definition exp3 (e: nat): int := ff_exp (Int.repr 3) (Int.repr (Z.of_nat e)).

Lemma exp3_S: forall n,
(0 <= n <= 256)%nat -> (* even holds for bigger n, as long as no int overflow *)
exp3 (S n) = ff_mult (Int.repr 3) (exp3 n).