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(* Schema d'axiome de comprehension                                        *)
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Parameter subset : (E -> Prop) -> E -> E.Axiom
  axs_comprehension :
    forall (F : E -> Prop) (a v0 : E), In v0 (subset F a) <-> In v0 a /\ F v0.

Lemma lem_comprehension_rl : forall (F : E -> Prop) (a v0 : E), In v0 a /\ F v0 -> In v0 (subset F a).