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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 _close_remaining_channels(self):
 """Forcefully close all open channels. :return: """

        for channel_id in list(self._channels):
            self._channels[channel_id].set_state(Channel.CLOSED)
            self._channels[channel_id].close()
            self._cleanup_channel(channel_id)