(** [log2_up] n is strictly positive for 1<n *)

(** [log2_up] and multiplication. *)
(** Due to rounding error, we don't have the usual [log2_up (a*b) = log2_up a + log2_up b] but we may be off by 1 at most *)
Lemma log2_up_mul_above : forall a b, 0<=a -> 0<=b ->
log2_up (a*b) <= log2_up a + log2_up b.