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 Select_SplitSelect16Q_Zlength Q i front back:
(front, back) = SplitSelect16Q Q i -> 0<= i < 4 ->
Zlength front = i /\ Zlength back = 3-i.