code: def level_at_index(self, index):
          
          if not isinstance(index, int):
              self.log_exc(u"Index is not an integer", None, True, TypeError)
          levels = self.levels
          if (index < 0) or (index >= len(levels)):
              self.log_exc(u"The given level index '%d' is not valid" % (index), None, True, ValueError)
          return self.levels[index]
docs: Return the list of nodes at level ``index``,
          in DFS order.
  
          :param int index: the index
          :rtype: list of :class:`~aeneas.tree.Tree`
  
          :raises: ValueError if the given ``index`` is not valid