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