Fixpoint extendr {A} (l : list A) ls :=
match l, ls with
| x :: xs, y :: ys => (y ++ [x]) :: extendr xs ys
| _, _ => ls
end.

Lemma data_at__eq : forall {cs : compspecs} sh t p, data_at_ sh t p = data_at sh t (default_val t) p.