proposition: Lemma zspan_uniq xmin xmax : uniq (zspan xmin xmax).
proof: Proof. rewrite /zspan /zwidth; case: {xmax}(xmax - xmin) => -[|n] //. apply/(sorted_uniq lt_trans ltxx)=> /=; set w := traject _ _ _. have: fpath (+%R^~ 1) xmin w by apply: fpath_traject. by apply/sub_path=> x _ /eqP<-; rewrite ltr_addl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)