(* End of proof of lem_Einter_vide                                         *)
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(* Complementaire d'un ensemble                                            *)
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Definition Comp (a b : E) := subset (fun y : E => ~ In y b) a.Parameter OComp : E -> E -> E.Axiom def_OComp : forall a b y : E, In y (OComp a b) <-> In y a /\ ~ In y b.

(* Specifications: *)
Lemma lem_Comp_eq_OComp : forall a b : E, OComp a b = Comp a b.