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