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

Lemma mapsto_exclusive : forall (sh : Share.t) (t : type) (v : val), sepalg.nonunit sh -> exclusive_mpred (EX v2 : _, mapsto sh t v v2).