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

Lemma field_at__exclusive : forall (cs : compspecs) (sh : Share.t) (t : type) (fld : list gfield) (p : val), sepalg.nonidentity sh -> 0 < sizeof (nested_field_type t fld) -> exclusive_mpred (field_at_ sh t fld p).