intros. rewrite skipn_app1.
specialize (skipn_exact_length (force_lengthn n l a)).
rewrite force_lengthn_length_n. intros X; rewrite X; trivial.
rewrite force_lengthn_length_n; omega.
Qed.

Lemma data_at_triv {cs} sh t v v': v=v' -> @data_at cs sh t v |-- @data_at cs sh t v'.