GtkComboBox can show tree structure? how?

[Olivier Sessink <lists olivier pk wau nl>]

According to the docs "The tree model holding the valid choices is not 
restricted to a flat list, it can be a real tree, and the popup will 
reflect the tree structure."

I have data in a treestore, and it is indeed a tree structure. But all 
data show up as one list, no nesting, nu submenu,  nothing. What it is I 
should  add to this code?

thanks,
        Olivier

GtkCellRenderer *renderer;
fb2->dirmenu_v = gtk_combo_box_new_with_model(fb2->dirmenu_filter);
renderer = gtk_cell_renderer_pixbuf_new();
gtk_cell_layout_pack_start(GTK_CELL_LAYOUT(fb2->dirmenu_v),renderer,FALSE);
gtk_cell_layout_set_attributes(GTK_CELL_LAYOUT(fb2->dirmenu_v), renderer
                , "pixbuf", PIXMAP_COLUMN
                , "pixbuf_expander_closed", PIXMAP_COLUMN
                , "pixbuf_expander_open", PIXMAP_COLUMN
                , NULL);
renderer = gtk_cell_renderer_text_new();
gtk_cell_layout_pack_start(GTK_CELL_LAYOUT(fb2->dirmenu_v),renderer, TRUE);
gtk_cell_layout_set_attributes(GTK_CELL_LAYOUT(fb2->dirmenu_v), renderer
                , "text", FILENAME_COLUMN
                , NULL);