proposition: Lemma ler_dist_normD v w : `| `|v| - `|w| | <= `|v + w|.
proof: Proof. by rewrite -[w]opprK normrN ler_dist_dist. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)