The treemodel.py example does not cover all the cases which
a real tree model should cover.

For instance the following change should not break the
example, which it does.

          model = MyTreeModel()
+         model = model.filter_new()
          tree_view = gtk.TreeView(model)

It can be fixed by checking the special case of iter==None
in every method, but it's unclear if that is overkill.