fact: Parameter sub_DN_correct : forall p x y, valid_lb x = true -> valid_ub y = true -> (valid_lb (sub_DN p x y) = true /\ le_lower (toX (sub_DN p x y)) (Xsub (toX x) (toX y))).
imports: Coq(ZArith Reals) Flocq(Zaux Raux) Xreal Basic