instance (Monad m) => Applicative (StateT s m) where
  pure a = StateT $ \s -> pure $ (a, s)
  (<*>) :: (StateT s m (a -> b)) -> (StateT s m a) -> (StateT s m b)
  (StateT smf) <*> (StateT sma) = StateT $ \s -> do
    (f, s1) <- smf s
    (a, s2) <- sma s
    return (f a, s2)