code: def generate_classes(self, items):
          "Generate classes from unique `items` and add `background`."
          classes = super().generate_classes([o[1] for o in items])
          classes = ['background'] + list(classes)
          return classef generate_classes(self, items):
          "Generate classes from unique `items` and add `background`."
          classes = super().generate_classes([o[1] for o in items])
          classes = ['background'] + list(classes)
          return classes
docs: Generate classes from unique `items` and add `background`.