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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 join(self):
 """Wait for transfer to exit, raising errors as necessary."""

        self.closed = True

        while self.expect > 0:
            val = self.wait_change.get()
            self.expect -= 1

            if val is not None:
                # Wait a while for all running greenlets to exit, and
                # then attempt to force them to exit so join()
                # terminates in a reasonable amount of time.
                gevent.joinall(list(self.greenlets), timeout=30)
                gevent.killall(list(self.greenlets), block=True, timeout=30)
                raise val