Proof using.
intros. subst.
destruct l2; eauto using app_nil_r.
simpl in *.
break_match; intuition. subst. simpl in *.
unfold contiguous_range_exact_lo in *.
intuition. specialize (H0 e0). conclude_using intuition.
omega.
Qed.

Lemma sorted_app_1 :
forall l1 l2,
sorted (l1 ++ l2) ->
sorted l1.