destruct n; intuition; simpl in *.
eapply app_eq_nil in H.
intuition.
discriminate.
Qed.

(* Specifications: *)
Theorem firstn_allNatsLt_h :
forall ls n1 n2,
n2 >= n1 ->
ls = (allNatsLt n2) ->
firstn n1 ls = allNatsLt n1.