zip with index on rose tree

NT.hs

data RT a = N a [RT a] deriving (Show)
no a ns = N a ns
lea a = N a []

sz (N _ cs) = length cs

l (N a cs) = foldl (\accu b -> b) a (l <$> cs) 

slast l = if length l > 0 then Just (last l) else Nothing

inol :: RT a -> RT (Int, a)
inol = inol' 0
  where
    inol' :: Int -> RT a -> RT (Int, a)
    inol' s t@(N a cs) = let ls = take (sz t `div` 2) cs
                             rs = drop (sz t `div` 2) cs
                             ls' = foldr (\a accu -> accu ++ [(inol' (maybe s (1 +) (fst . l <$> slast accu)) a)]) [] ls
                             nn = (maybe s (1 +) (fst . l <$> slast ls'))
                             rs' = foldr (\a accu -> accu ++ [(inol' (1 + maybe nn (fst) (l <$> slast accu)) a)]) [] rs
                         in
                           N (nn, a) $ ls' ++ rs'