proposition: Lemma eq_minus' : forall a b c : nat, c <= a -> c <= b -> a - c = b - c -> a = b.
proof: Proof. intros. rewrite <- (Nat.sub_add c a); auto with arith. rewrite H1. rewrite Nat.sub_add; auto. Qed.
imports: Coq(Arith ArithRing)