fact: Fixpoint Nwadd (w1 w2 : Nword) {struct w1} : Nword := match w1, w2 with | n1::w1', n2:: w2' => (n1+n2)%N :: Nwadd w1' w2' | nil, _ => w2 | _, nil => w1 end.
imports: Zbool BinInt BinNat Setoid Ring_theory Ring_polynom