Definition DMerge :=
    MapFold (Map A) (Map A) (M0 A) (MapMerge A) (fun (_:ad) (m:Map A) => m).

Lemma in_dom_DMerge_1 : forall (m:Map (Map A)) (a:ad), in_dom A a (DMerge m) = match MapSweep _ (fun (_:ad) (m0:Map A) => in_dom A a m0) m with | Some _ => true | _ => false end.