Definition HTrue_inv intsums xs ys:Prop:=
Zlength intsums = 16 /\
        (forall j, 0 <= j < 16 ->
           exists xj, exists yj,
           Znth j (map Vint xs) = Vint xj /\
           Znth j (map Vint ys) = Vint yj /\
           Znth j (map Vint intsums) = Vint (Int.add yj xj)).

Lemma HTrue_inv_char l xs ys: Zlength xs = 16 -> Zlength ys=16 -> HTrue_inv l xs ys -> Some l = sumlist xs ys.