(** same trick for [equal] *)

  Definition equal (A:Type)(cmp:A -> A -> bool)(m m' : t A) : bool := 
    L.equal cmp (elements m) (elements m').

Lemma equal_2 : forall (A:Type)(m: t A)(m': t A)(cmp: A -> A -> bool), equal cmp m m' = true -> Equivb cmp m m'.