Remark exists_for_all:
  forall m f,
  exists_ m f = negb (for_all m (fun x a => negb (f x a))).Remark for_all_exists:
  forall m f,
  for_all m f = negb (exists_ m (fun x a => negb (f x a))).

Lemma exists_false: forall m f, exists_ m f = false <-> (forall x a, T.get x m = Some a -> f x a = false).