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