proposition: Theorem intro_Z : forall n:nat, exists y : Z, Z.of_nat n = y /\ 0 <= y * 1 + 0.
proof: Proof. intros n; exists (Z.of_nat n); split; trivial. rewrite Z.mul_1_r, Z.add_0_r. apply Nat2Z.is_nonneg. Qed.
imports: BinInt Znat