proposition: Lemma size_zspan xmin xmax : size (zspan xmin xmax) = zwidth xmin xmax.
proof: Proof. by rewrite /zspan size_traject. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)