proposition: Lemma distrC v w : `|v - w| = `|w - v|.
proof: Proof. by rewrite -opprB 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)