(* works, but very slow! Lemma exp_table_correct : forall a : nat, (a < 256)%nat -> nth a ff_exp_table Int.zero = exp3 a. *)

Lemma fast_times3: forall (i : int),
0 <= (Int.unsigned i) < 256 ->
Int.xor i (xtime i) = ff_mult (Int.repr 3) i.