proposition: Lemma oppRD x y : - (x + y) == - x - y.
proof: Proof. by apply: (@addRI (x + y)); rewrite subRR addRCA addRK addRC subRR. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div order) mathcomp(ssralg ssrnum ssrint rat intdiv) Coq(Morphisms Setoid) fourcolor(real realsyntax)