Inductive powers (a : U) : Ensemble U :=
    In_powers : forall (m : Z) (x : U), x = exp m a -> In U (powers a) x.Inductive powers (a : U) : Ensemble U :=
    In_powers : forall (m : Z) (x : U), x = exp m a -> In U (powers a) x.

Lemma Ex3 : forall n : Z, exp n e = e.