proposition: Lemma subR_ge0 x y : 0 <= y - x <-> x <= y.
proof: Proof. by rewrite -(leR_add2r x) subRK add0R. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div order) mathcomp(ssralg ssrnum ssrint rat intdiv) Coq(Morphisms Setoid) fourcolor(real realsyntax)