proposition: Theorem spec_of_Z: forall x, to_Z (of_Z x) = x.
proof: Proof. intros x; case x; unfold to_Z, of_Z, zero. exact NN.spec_0. intros; rewrite NN.spec_of_N; auto. intros; rewrite NN.spec_of_N; auto. Qed.
imports: ZArith Lia BigNumPrelude NSig ZSig