{-# LANGUAGE DeriveFunctor #-}

import Data.List

data Tree a = Node a [Tree a] deriving (Functor, Show)

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

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

scan f (Node r l) = Node r $ map (fmap (f r) . scan f) l

main = print $ foldl' (+) 0 $ take 1800 $ flatten_a $ scan (+) a