Definition Equivb m m' :=
  (forall k, In k m <-> In k m') /\
  (forall k e e', MapsTo k e m -> MapsTo k e' m' -> cmp e e' = true).

Lemma map_option_2 : forall (m:t elt)(x:key), In x (map_option f m) -> exists d, MapsTo x d m /\ f x d <> None.