Notation eqke := (@eq_key_elt elt).
Notation eqk := (@eq_key elt).
Instance eqk_equiv : Equivalence eqk.
(** Complements about InA, NoDupA and findA *)

Lemma NoDupA_eqk_eqke : forall l, NoDupA eqk l -> NoDupA eqke l.