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

Lemma in_dom_DMerge_3 : forall (m:Map (Map A)) (a b:ad) (m0:Map A), MapGet _ m a = Some m0 -> in_dom A b m0 = true -> in_dom A b (DMerge m) = true.