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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 dumps(self, fd, **kwargs):
 """ Returns the concrete content for a file descriptor. BACKWARD COMPATIBILITY: if you ask for file descriptors 0 1 or 2, it will return the data from stdin, stdout, or stderr as a flat string. :param fd: A file descriptor. :return: The concrete content. :rtype: str """

        if 0 <= fd <= 2:
            data = [self.stdin, self.stdout, self.stderr][fd].concretize(**kwargs)
            if type(data) is list:
                data = b''.join(data)
            return data
        return self.get_fd(fd).concretize(**kwargs)