':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 previous node at the current level.
If there is no previous `iter`, :obj:`None` is returned.'

def iter_previous(self, iter):

    prev_iter = iter.copy()
    success = super(TreeModel, self).iter_previous(prev_iter)
    if success:
        return prev_iter
