proposition: Lemma ler_normB v w : `|v - w| <= `|v| + `|w|.
proof: Proof. by rewrite (le_trans (ler_normD _ _)) ?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)