(>>=@) :: B a -> (a -> UI b) -> UI (B b)
ba >>=@ f = do
  (e,h) <- newEvent
  onChanges ba $ f >=> h
  a <- f =<< currentValue ba
  stepper a e
(>>=@) :: B a -> (a -> UI b) -> UI (B b)
ba >>=@ f = do
  (e,h) <- newEvent
  onChanges ba $ f >=> h
  a <- f =<< currentValue ba
  stepper a e
ba >>=@ f = do
  (e,h) <- newEvent
  onChanges ba $ f >=> h
  a <- f =<< currentValue ba
  stepper a e