def set_stream(self, stream_name: str):
        """
        Switches to a specific stream.

        :param stream_name: The name of the stream.
        """
        self._current_stream = stream_name
        if not self._is_stream_tracked(stream_name):
            return

        if self._current_stream not in self._amcas:
            self._amcas[stream_name] = AverageMeanClassAccuracy(
                classes=self._limit_classes
            )
        self._streams_in_this_phase.add(stream_name)

        Switches to a specific stream.

        :param stream_name: The name of the stream.
        
avalanche/evaluation/metrics/amca.py