code: def feature_uri(uri):
      
      def decorator(class_):
          """Returns a decorated class"""
          if "_pyxmpp_feature_uris" not in class_.__dict__:
              class_._pyxmpp_feature_uris = set()
          class_._pyxmpp_feature_uris.add(uri)
          return class_
      return decorator
docs: Decorating attaching a feature URI (for Service Discovery or Capability
      to a XMPPFeatureHandler class.