(* End of proof of lem_sig_equiv_fun_make                                  *)

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(* Projection sur le quotient                                              *)
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Definition projQuotient (r a : E) (rRel : rel_sig r a a)
  (rEqv : equivalence r a rRel) (rD : rdom r a a rRel = a) :=
  subset
    (fun x : E =>
     exists z : E,
       (exists t : E, x = couple z t /\ t = equivClass r a rRel rEqv z))
    (cartesien a (quotient r a rRel rEqv rD)).

Lemma lem_x_equiv_in_projQ : forall (r a : E) (rRel : rel_sig r a a) (rEqv : equivalence r a rRel) (rD : rdom r a a rRel = a) (x : E), In x a -> In (couple x (equivClass r a rRel rEqv x)) (projQuotient r a rRel rEqv rD).