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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 subscribe(self, client, channel_name):
 """Register a new client to receive messages on a channel."""

        if channel_name not in self.channels:
            self.channels[channel_name] = channel = Channel(channel_name)
            channel.start()

        self.channels[channel_name].subscribe(client)