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 isptrD v: isptr v -> exists b ofs, v = Vptr b ofs.