code: def register_filter(filter_cls):
      
      if filter_cls.name is None:
          return
      elif filter_cls.name in FILTER_REGISTRY:
          raise RuntimeError(
              "Filter class already registered: {}".format(filter_cls.name))
      else:
          FILTER_REGISTRY[filter_cls.name] = filter_cls
docs: Adds the ``filter_cls`` to our registry.