code: def register_feature(self, var):
          
          if var in self._features or var in self.STATIC_FEATURES:
              raise ValueError("feature already claimed: {!r}".format(var))
          self._features.add(var)
          self.on_info_changed()
docs: Register a feature with the namespace variable `var`.
  
          If the feature is already registered or part of the default :xep:`30`
          features, a :class:`ValueError` is raised.