proposition: Lemma subR_le0 x y : x - y <= 0 <-> x <= y.
proof: Proof. by rewrite -subR_ge0 oppRB add0R subR_ge0. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div order) mathcomp(ssralg ssrnum ssrint rat intdiv) Coq(Morphisms Setoid) fourcolor(real realsyntax)