proposition: Theorem add_xals : forall a b c : R, forall zi : FF, BND ((a - c) + (c + b)) zi -> BND (a + b) zi.
proof: Proof. intros a b c zi Hz. replace (a + b)%R with ((a - c) + (c + b))%R. exact Hz. ring. Qed.
imports: Reals Gappa_common