repeat rewrite Zlength_map.  simpl.
repeat rewrite Zlength_app. repeat rewrite <- QuadByteByteList_ZLength.
reflexivity. Qed.
Definition SByte (q:SixteenByte) (v:val) : mpred :=
@data_at CompSpecs Tsh (Tarray tuchar 16 noattr) (SixteenByte2ValList q) v.

Lemma ThirtyTwoByte_split16 q v:
field_compatible (Tarray tuchar 32 noattr) [] v ->
ThirtyTwoByte q v =
(SByte (fst q) v * SByte (snd q) (offset_val 16 v))%logic.