Require Export TopologicalSpaces Ensembles InteriorsClosures.
Definition open_neighborhood {X:TopologicalSpace}
(U:Ensemble (point_set X)) (x:point_set X) :=
open U /\ In U x.
Definition neighborhood {X:TopologicalSpace}
(N:Ensemble (point_set X)) (x:point_set X) :=
exists U:Ensemble (point_set X),
open_neighborhood U x /\ Included U N.

Lemma open_neighborhood_is_neighborhood: forall {X:TopologicalSpace}
(U:Ensemble (point_set X)) (x:point_set X),
open_neighborhood U x -> neighborhood U x.