Qed.
Notation pow2 := (Nat.pow 2).
Fixpoint Npow2 (n : nat) : N :=
match n with
| O => 1
| S n' => 2 * Npow2 n'
end%N.

(* Specifications: *)
Theorem untimes2 : forall n, n + (n + 0) = 2 * n.
auto.