code: def start(self, 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()
docs: Begin transfer for an indicated wal segment.