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 inductive_invariant:forall m, m<= n -> {B':t| Subset B' town /\ cardinal B' = m /\ forall b'0 b'1, In b'0 B' -> In b'1 B' -> ~(E.eq b'0 b'1) -> knows b'0 b'1}.