End MapFoldExists.
Section DMergeDef.
Variable A : Type.
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.