code: def on_config_inited(app, config):
      
      extension_paths = config["uqbar_book_extensions"] or [
          "uqbar.book.extensions.GraphExtension"
      ]
      app.uqbar_book_extensions = []
      for extension_path in extension_paths:
          module_name, _, class_name = extension_path.rpartition(".")
          module = importlib.import_module(module_name)
          extension_class = getattr(module, class_name)
          extension_class.setup_sphinx(app)
          app.uqbar_book_extensions.append(extension_class)
docs: Hooks into Sphinx's ``config-inited`` event.