code: def register_renderer(app, id, renderer, force=True):
      
      renderers = app.extensions.setdefault('nav_renderers', {})
  
      if force:
          renderers[id] = renderer
      else:
          renderers.setdefault(id, renderer)
docs: Registers a renderer on the application.
  
      :param app: The :class:`~flask.Flask` application to register the renderer
                  on
      :param id: Internal id-string for the renderer
      :param renderer: Renderer to register
      :param force: Whether or not to overwrite the renderer if a different one
                    is already registered for ``id``