(* End of proof of lem_Comp_eq_OComp                                       *)

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(* Difference symetrique de deux ensembles                                 *)
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Definition Delta (a b : E) := union (Comp a b) (Comp b a).

Lemma lem_union_dist : forall a b c : E, union a (inter b c) = inter (union a b) (union a c).