Definition SByte (q:SixteenByte) (v:val) : mpred :=
  @data_at CompSpecs Tsh (Tarray tuchar 16 noattr) (SixteenByte2ValList q) v.

Lemma QuadByte2ValList_firstn4 q l: firstn 4 (QuadByte2ValList q ++ l) = QuadByte2ValList q.