proposition: Lemma addRI x y z : x + y == x + z -> y == z.
proof: Proof. by move=> Dxy; rewrite -(addKR x y) Dxy addKR. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div order) mathcomp(ssralg ssrnum ssrint rat intdiv) Coq(Morphisms Setoid) fourcolor(real realsyntax)