Definition X_content (x: SixteenByte * SixteenByte * (SixteenByte * SixteenByte))
                     (i:Z) (l:list val) : Prop :=
    l = upd_upto x (Z.to_nat i) (list_repeat 16 Vundef).

Lemma XcontUpdate Nonce C Key1 Key2 i l (I: 0 <= i < 4) (L: X_content (Nonce, C, (Key1, Key2)) i l): X_content (Nonce, C, (Key1, Key2)) (i + 1) (upd_Znth (11 + i) (upd_Znth (6 + i) (upd_Znth (1 + i) (upd_Znth (5 * i) l (Vint (littleendian (Select16Q C i)))) (Vint (littleendian (Select16Q Key1 i)))) (Vint (littleendian (Select16Q Nonce i)))) (Vint (littleendian (Select16Q Key2 i)))).