End my_MapFold.
(* Next we show the assumptions in the previous section are satisfied if M is a set of maps, neutral is the empty map, op is MapMerge and R is eqmap *)

(* MapMerge is associative *)
Lemma MapMerge_assoc :
forall (A : Set) (a b c : Map A),
eqmap _ (MapMerge _ (MapMerge _ a b) c) (MapMerge _ a (MapMerge _ b c)).