proposition: Lemma free_coports_card : (#|free_coports| > 0)%nat.
proof: Proof. rewrite /free_coports cardsCp. move: (card_in_imset (@edges_inj _ c)). rewrite (edom_codom c) /edom => ->. rewrite -cardsCp card_gt0; apply /set0Pn. exists p. by rewrite setCD in_setU Hpb orbT. Qed.
imports: mathcomp(all_ssreflect ssralg fingroup zmodp poly ssrnum) mathcomp(matrix perm) ssr_ext ssralg_ext fdist subgraph_partition tanner