instance Monad []  where
    {-# INLINE (>>=) #-}
    xs >>= f             = [y | x <- xs, y <- f x]
    {-# INLINE (>>) #-}
    (>>) = (*>)
    {-# INLINE return #-}
    return x            = [x]
    {-# INLINE fail #-}
    fail _              = []