proposition: Lemma card_transversal : #|X| = #|P|.
proof: Proof. by rewrite -pblock_transversal card_in_imset //; apply: pblock_inj. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)