Inductive Power_set (A : Ensemble U) : Ensemble (Ensemble U) :=
       Definition_of_Power_set :
         forall X : Ensemble U,
         Included U X A -> In (Ensemble U) (Power_set A) X.Inductive Power_set (A : Ensemble U) : Ensemble (Ensemble U) :=
       Definition_of_Power_set :
         forall X : Ensemble U,
         Included U X A -> In (Ensemble U) (Power_set A) X.

Theorem Non_disjoint_union : forall (X : Ensemble U) (x : U), In U X x -> Union U (Singleton U x) X = X.