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我正在尝试为 Cofree 编写以下 Monoid 实例：

instance (Monoid a, Monoid (f (Cofree f a))) => Monoid (Cofree f a) where
  mempty = mempty :< mempty
  (a :< rest) `mappend` (b :< rest') = (a `mappend` b) :< (rest `mappend` rest')

但我收到以下错误：

• Variable ‘f’ occurs more often
    in the constraint ‘Monoid (f (Cofree f a))’
    than in the instance head
  (Use UndecidableInstances to permit this)
• In the instance declaration for ‘Monoid (Cofree f a)’

我以前遇到过 Undecidable Instances ，但我不确定为什么这是不可判定的，我该如何解决？我需要添加 UndecidableInstances 吗？谢谢！！

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