data Tree a
  = Node (Tree a) a (Tree a)
  | Leaf

data A f = A1 (f ()) | A2 (f (Tree ()))

data Either a b = Left a | Right b

data ExceptT e m a = ExceptT (Either e (m a))

data TreeF a r
  = NodeF r a r
  | LeafF

data Fix f = In (f (Fix f))

data Tree' a = Tree' (Fix (TreeF a))