proposition: Lemma addC x y : x + y == y + x.
proof: Proof. by split=> a [b ltxb ltyab]; exists (a - b)%R; rewrite // opprD addNKr opprK. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq order) mathcomp(ssralg ssrnum ssrint rat) fourcolor(real) Coq(Setoid Morphisms)