Add Parametric Relation {A} : (Ensemble A) Same_set
    reflexivity proved by (Same_set_refl A)
    symmetry proved by (Same_set_sym A)
    transitivity proved by (Same_set_trans A) as Same_set_rel.

Lemma Union_assoc: forall A P Q R, Same_set (Union A (Union A P Q) R) (Union A P (Union A Q R)).