Note that the Executor must be close()'d elsewhere,
		or join() will never return.

def join(self):
		"""Note that the Executor must be close()'d elsewhere,
		or join() will never return.
		"""
		self.inputfeeder_thread.join()
		self.pool.join()
		self.resulttracker_thread.join()
		self.failuretracker_thread.join()