data BinTree a = BNode (BinTree a) a (BinTree a)
               | BEmpty
               deriving ( Eq, Show, Read )

data Direction = L | R
               deriving ( Eq, Ord, Show, Read )
modify :: BinTree a -> [Direction] -> a -> BinTree a
modify = undefined

data TreeDir a = TLeft a (BinTree a)
               | TRight (BinTree a) a
               deriving ( Eq, Show, Read )

data TreeZipper a = TZip [TreeDir a] (BinTree a)
                  deriving ( Eq, Show, Read )

toZipper :: BinTree a -> TreeZipper a
toZipper tree = TZip [] tree

fromZipper :: TreeZipper a -> BinTree a
fromZipper (TZip [] tree) = tree
fromZipper (TZip (TLeft v r : ds) tree) =
            fromZipper $ TZip ds (BNode tree v r)
fromZipper (TZip (TRight l v : ds) tree) =
            fromZipper $ TZip ds (BNode l v tree)