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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 start(self, segment):
 """Begin transfer for an indicated wal segment."""


        if self.closed:
            raise UserCritical(msg='attempt to transfer wal after closing',
                               hint='report a bug')

        g = gevent.Greenlet(self.transferer, segment)
        g.link(self._complete_execution)
        self.greenlets.add(g)

        # Increment .expect before starting the greenlet, or else a
        # very unlucky .join could be fooled as to when pool is
        # complete.
        self.expect += 1

        g.start()