(* Decidable Dedekind cuts are Cauchy reals. *)
Record DedekindDecCut : Type :=
{
DDupcut : Q -> Prop;
DDproper : forall q r : Q, (q == r -> DDupcut q -> DDupcut r)%Q;
DDlow : Q;
DDhigh : Q;
DDdec : forall q:Q, { DDupcut q } + { ~DDupcut q };
DDinterval : forall q r : Q, Qle q r -> DDupcut q -> DDupcut r;
DDhighProp : DDupcut DDhigh;
DDlowProp : ~DDupcut DDlow;
}.

(* Specifications: *)
Lemma DDlow_below_up : forall (upcut : DedekindDecCut) (a b : Q),
DDupcut upcut a -> ~DDupcut upcut b -> Qlt b a.