Definition compact (X:TopologicalSpace) :=
  forall C:Family (point_set X),
    (forall U:Ensemble (point_set X), In C U -> open U) ->
    FamilyUnion C = Full_set ->
    exists C':Family (point_set X),
      Finite _ C' /\ Included C' C /\
      FamilyUnion C' = Full_set.

Lemma compact_impl_net_cluster_point: forall X:TopologicalSpace, compact X -> forall (I:DirectedSet) (x:Net I X), inhabited (DS_set I) -> exists x0:point_set X, net_cluster_point x x0.