Definition a_equiv (l1 : list (K * V)) l2 :=
    forall k,
      assoc l1 k = assoc l2 k.

(* Specifications: *)
Lemma a_equiv_refl : forall l, a_equiv l l.
Lemma a_equiv_sym : forall l l', a_equiv l l' -> a_equiv l' l.
Lemma a_equiv_trans : forall l l' l'', a_equiv l l' -> a_equiv l' l'' -> a_equiv l l''.