Definition F (A : Set) (f : ad -> A -> M) (r : ad * A) 
  (m : M) := let (a, y) := r in op (f a y) m.

Lemma MapMerge_eq : forall (A : Set) (a a1 b b1 : Map A), eqmap _ a a1 -> eqmap _ b b1 -> eqmap _ (MapMerge _ a b) (MapMerge _ a1 b1).