code: def iter_previous(self, iter):
          
  
          prev_iter = iter.copy()
          success = super(TreeModel, self).iter_previous(prev_iter)
          if success:
              return prev_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 previous node at the current level.
  
          If there is no previous `iter`, :obj:`None` is returned.