code: def anchor_stream(self, stream_id, converter="rtc"):
          
  
          if isinstance(converter, str):
              converter = self._known_converters.get(converter)
  
              if converter is None:
                  raise ArgumentError("Unknown anchor converter string: %s" % converter,
                                      known_converters=list(self._known_converters))
  
          self._anchor_streams[stream_id] = converter
docs: Mark a stream as containing anchor points.