code: def _find_path(start, end, path=tuple()):
      
      path = path + (start, )
      if start is end:
          return path
      else:
          ret = None
          if start.lo is not None:
              ret = _find_path(start.lo, end, path)
          if ret is None and start.hi is not None:
              ret = _find_path(start.hi, end, path)
          return ret
docs: Return the path from start to end.
  
      If no path exists, return None.