rewrite mult_0_r; auto with arith.
Qed.

(* Specifications: *)
Theorem gt_mult_rev_0_l: forall a b, a * b > 0 ->  a > 0.
intros a b; case a; simpl; auto with arith.
Theorem le_0_eq_0: forall n, n <= 0 -> n = 0.
intros n; case n; auto with arith.