code: def fill_tree(comments):
      
      if not comments:
          return
  
      it = iter(comments)
      first = next(it)
      extra_path_items = imap(_mark_as_root_path, first.root_path)
      return chain(extra_path_items, [first], it)
docs: Insert extra comments in the comments list, so that the root path of the first comment is always visible.
      Use this in comments' pagination to fill in the tree information.
  
      The inserted comments have an ``added_path`` attribute.