proposition: Lemma lerB_normD v w : `|v| - `|w| <= `|v + w|.
proof: Proof. by rewrite -{1}[v](addrK w) lterBDl (le_trans (ler_normD _ _))// addrC normrN. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)