induction n; intuition; simpl in *.
rewrite plus_0_r.
assert (S n = 1 + n)%nat.
omega.
rewrite H.
eapply plus_le_compat.
eapply expnat_2_ge_1.
trivial.
Qed.

Lemma expnat_1 : forall k,
expnat 1%nat k = 1%nat.