Inductive separable (X:TopologicalSpace) : Prop :=
  | intro_dense_ctbl: forall S:Ensemble (point_set X),
    Countable S -> dense S -> separable X.Inductive separable (X:TopologicalSpace) : Prop :=
  | intro_dense_ctbl: forall S:Ensemble (point_set X),
    Countable S -> dense S -> separable X.Definition Lindelof (X:TopologicalSpace) : Prop :=
  forall cover:Family (point_set X),
    (forall U:Ensemble (point_set X),
       In cover U -> open U) ->
    FamilyUnion cover = Full_set ->
  exists subcover:Family (point_set X), Included subcover cover /\
     Countable subcover /\ FamilyUnion subcover = Full_set.Inductive second_countable (X:TopologicalSpace) : Prop :=
  | intro_ctbl_basis: forall B:Family (point_set X),
    open_basis B -> Countable B -> second_countable X.Inductive second_countable (X:TopologicalSpace) : Prop :=
  | intro_ctbl_basis: forall B:Family (point_set X),
    open_basis B -> Countable B -> second_countable X.

Lemma second_countable_impl_first_countable: forall X:TopologicalSpace, second_countable X -> first_countable X.