Definition Cardinality (F: finType) := | elem F |.
(** * Dupfreeness *)
(* Proofs about dupfreeness *)

Lemma dupfree_concat (X: Type) (A: list (list X)) : (forall B, B el A -> dupfree B) /\ (forall B C, B <> C -> B el A -> C el A -> disjoint B C) -> dupfree A -> dupfree (concat A).