proposition: Theorem sub_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