proposition: Lemma step_border n (p : port) : border n :\ p \subset ports n.
proof: Proof. eapply subset_trans; [apply subD1set | apply border_p]. Qed.
imports: mathcomp(all_ssreflect ssralg fingroup zmodp poly ssrnum) mathcomp(matrix perm) ssr_ext ssralg_ext fdist subgraph_partition tanner