Module NatSet := FSetList.Make(OrderedTypeEx.Nat_as_OT).Infix "++" := add (at level 60, right associativity).Notation "s [=] t" := (Equal s t) (at level 70, no associativity).Module GeneralProperties := FSetProperties.Properties NatSet.

Lemma extendible_to_n:forall B', Subset B' town -> cardinal B' <= n -> {B:t| cardinal B = n /\ Subset B' B /\ Subset B town}.