Opaque sizeof.

Lemma split2_data_at_Tarray_app:
forall {cs: compspecs} mid n (sh: Share.t) (t: type)
(v1 v2: list (reptype t)) p,
Zlength v1 = mid ->
Zlength v2 = n-mid ->
data_at sh (tarray t n) (v1 ++ v2) p =
data_at sh (tarray t mid) v1  p *
data_at sh (tarray t (n-mid)) v2
(field_address0 (tarray t n) [ArraySubsc mid] p).