A tree fold first folds every child rose tree, then supplies the resulting list and the current label to the combining function:
foldRTree :: (a -> [b] -> b) -> RTree a -> b
foldRTree f (Node x children) =
  f x (map (foldRTree f) children)
The label type a and result type b are independent, so the polymorphic type needs no class constraint. With f x ys = x + sum ys, every label is counted exactly once; the illustrated labels sum to 28.
The printed source gives the signature exampleRTree but binds exampleRTRee, with different capitalization. They are different Haskell names; the demonstration requires using one spelling consistently. This typo does not affect the tree fold definition.

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