def __init__(self): return TemplateSyntaxError.__init__( self, ("The 'sub_menu' tag can only be used in menu templates to render " "parts of other menus, initiated by calling one of the other " "included tags: 'main_menu', 'flat_menu', 'section_menu' or " "'children_menu' (custom tags are not supported). You might " "want to update your template to use the 'children_menu' tag " "instead."))