proposition: Lemma ler_distD u v w : `|v - w| <= `|v - u| + `|u - w|.
proof: Proof. by rewrite (le_trans _ (ler_normD _ _)) // addrA addrNK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)