proposition: Lemma lerD2l x : {mono +%R x : y z / y <= z}.
proof: Proof. by move=> y z; rewrite -subr_ge0 opprD addrAC addNKr addrC subr_ge0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)