proposition: Lemma leR_add2l x y z : x + y <= x + z <-> y <= z.
proof: Proof. split=> leyz; last exact: (Real.add_monotone R). by rewrite -(addKR x y) -(addKR x z); apply: (Real.add_monotone R). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div order) mathcomp(ssralg ssrnum ssrint rat intdiv) Coq(Morphisms Setoid) fourcolor(real realsyntax)