fact: Record open_neighborhood_basis {X:TopologicalSpace} (NB:Family X) (x:point_set X) : Prop := { open_neighborhood_basis_elements: forall U:Ensemble X, In NB U -> open_neighborhood U x; open_neighborhood_basis_cond: forall U:Ensemble X, open_neighborhood U x -> exists V:Ensemble X, In NB V /\ Included V U }.
imports: TopologicalSpaces Neighborhoods OpenBases