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