(** 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'.