proposition: Lemma subRK x y : x - y + y == x.
proof: Proof. by rewrite addRAC addRK. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div order) mathcomp(ssralg ssrnum ssrint rat intdiv) Coq(Morphisms Setoid) fourcolor(real realsyntax)