gistfile1.hs

> bft                 =  unfoldr f
>                        where
>                        f [] = Nothing  
>                        f xs = Just (tops, concat subtrees)
>                          where
>                          (tops, subtrees) = unzip $ g xs 
>                            where  
>                            g ((Node l a r):xs) = (a, [l,r]):(g xs) 
>                            g (Empty:xs)        = g xs
>                            g []                = []