code: def iter_next(self, iter):
          
  
          next_iter = iter.copy()
          success = super(TreeModel, self).iter_next(next_iter)
          if success:
              return next_iter
docs: :param iter: the :obj:`Gtk.TreeIter`-struct
          :type iter: :obj:`Gtk.TreeIter`
  
          :returns: a :obj:`Gtk.TreeIter` or :obj:`None`
          :rtype: :obj:`Gtk.TreeIter` or :obj:`None`
  
          Returns an iterator pointing to the node following `iter` at the
          current level.
  
          If there is no next `iter`, :obj:`None` is returned.