intro. elim minus_n_O. rewrite minus_n_SO. simpl in |- *. auto with arith.
intros. simpl in |- *. exact H.
Qed.

(* Specifications: *)
Lemma minus_le_lem2 : forall a b : nat, a - b <= a.
simple induction b. elim minus_n_O. auto with arith.