proposition: Lemma inj_card_bij : bijective f.
proof: Proof. by exists (fun y => iinv (inj_card_onto y)) => y; rewrite ?iinv_f ?f_iinv. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path div)