proposition: Lemma distnEl n1 n2 : n2 <= n1 -> `|n1 - n2| = n1 - n2.
proof: Proof. by move/distnEr <-; rewrite distnC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)