proposition: Lemma zspan_dec_inc x n : zspan (x - 1) (x + n%:Z) = x - 1 :: traject (+%R^~ 1) x n.
proof: Proof. by rewrite /zspan /zwidth addrAC subKr /= subrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)