proposition: Lemma oppD x y : - (x + y) == - x - y.
proof: Proof. rewrite addC -[- x - y]add0 -(addN (y + x)) -addA [- _ + _]addC addA. by rewrite -(addA y) (addA x) !(addN, add0). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq order) mathcomp(ssralg ssrnum ssrint rat) fourcolor(real) Coq(Setoid Morphisms)