proposition: Lemma eqR_opp2 x y : opp x == opp y -> x == y.
proof: Proof. by move=> eq_nx_ny; rewrite -(oppK x) eq_nx_ny oppK. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq order) mathcomp(ssralg ssrnum ssrint rat) fourcolor(real) Coq(Setoid Morphisms)