':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.'

def iter_next(self, iter):

    next_iter = iter.copy()
    success = super(TreeModel, self).iter_next(next_iter)
    if success:
        return next_iter
