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