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data Free f a = Var a
| Node (f (Free f a))
instance Functor f => Functor (Free f) where
fmap g (Var a) = g a
fmap g (Node a) = fmap g a
instance Functor f => Monad (Free f) where
return a = Var a
(Var a) >>= g = g a
(Node a) >>= g = fmap g a
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