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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 closed(self, block_identifier: BlockSpecification) -> bool: """ Returns if the channel is closed. """

        return self.token_network.channel_is_closed(
            participant1=self.participant1,
            participant2=self.participant2,
            block_identifier=block_identifier,
            channel_identifier=self.channel_identifier,
        )