(* End of proof of lem_rel_prod_assoc                                      *)

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(* L'identite est un element neutre pour le produit                        *)
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(* Relations d'equivalence                                                 *)
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Definition equivalence (r a : E) (rRel : rel_sig r a a) :=
  reflexivity r a rRel /\ symmetry r a rRel /\ transitivity r a rRel.(***************************************************************************)
(* Classe d'equivalence                                                    *)
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Definition equivClass (r a : E) (rRel : rel_sig r a a)
  (rEqv : equivalence r a rRel) (x : E) :=
  subset (fun y : E => In (couple x y) r) a.

Lemma lem_equivClass_in_parties_a : forall (r a : E) (rRel : rel_sig r a a) (rEqv : equivalence r a rRel) (x : E), In (equivClass r a rRel rEqv x) (parties a).