Fixpoint MapFold1_state (state:State) (pf:ad -> ad) 
     (m:Map A) {struct m} : State * M :=
      match m with
      | M0 => (state, neutral)
      | M1 a y => f state (pf a) y
      | M2 m1 m2 =>
          match MapFold1_state state (fun a0:ad => pf (Ndouble a0)) m1 with
          | (state1, x1) =>
              match
                MapFold1_state state1
                  (fun a0:ad => pf (Ndouble_plus_one a0)) m2
              with
              | (state2, x2) => (state2, op x1 x2)
              end
          end
      end.Definition MapFold_state (state:State) :=
      MapFold1_state state (fun a:ad => a).

Lemma MapCollect_as_Fold : forall (f:ad -> A -> Map B) (m:Map A), MapCollect f m = MapFold (Map B) (M0 B) (MapMerge B) f m.