Definition SixteenWordRepI (w : SixteenWord) :=
  match w with (q0, q1, q2, q3) =>
    QuadWordRepI q0 ++ QuadWordRepI q1 ++ QuadWordRepI q2 ++ QuadWordRepI q3
  end.

Lemma QuadWordRepI_length s: length (QuadWordRepI s) = 4%nat.