proposition: Lemma length_nil A : List.length (@nil A) = 0%nat.
proof: Proof. reflexivity. Qed.
imports: Coq.ZArith.ZArith Coq.Lists.List Coq.Setoids.Setoid Coq.Classes.Morphisms Coq.QArith.QArith_base Coq.QArith.Qreduction Coq.QArith.Qround