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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def anchor_stream(self, stream_id, converter="rtc"):
 """Mark a stream as containing anchor points."""


        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