import Data.List

data Tree a = Node a [Tree a] deriving Show

a :: Tree Double
a = Node 1 [a]

flatten_a (Node x (n:_)) = x : flatten_a n

scan f a (Node x ns) = Node a' $ map (scan f a') ns where a' = f a x

main = print $ foldl' (+) 0 $ take 18000000 $ flatten_a $ scan (+) 0 a