In haskell, finish the impremention of `return` below.

class Arrow arr => ArrowApply arr where app :: (b `arr` c, b) `arr` c

newtype ArrowApply a => ArrowMonad a b = ArrowMonad (a () b)

instance ArrowApply a => Monad (ArrowMonad a) where return :: b -> ArrowMonad a b return = -- exercise

(>>=) :: ArrowMonad a a -> (a -> ArrowMonad a b) -> ArrowMonad a b (ArrowMonad a) >>= k = -- exercise

The 'ArrowApply' class is equivalent to 'Monad': any monad gives rise to a 'Kleisli' arrow, and any instance of 'ArrowApply' defines a monad.