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 QuadChunk2ValList_ZLength: forall l, Zlength (QuadChunks2ValList l) = (4 * Zlength l)%Z.