Definition Equal (m1 m2: T.t A) : Prop :=
  forall x, match T.get x m1, T.get x m2 with
                | None, None => True
                | Some a1, Some a2 => a1 === a2
                | _, _ => False
            end.

Lemma Equal_sym: forall m1 m2, Equal m1 m2 -> Equal m2 m1.