n. unregister
这种情况通常是bug的迹象——侦听器登记了，但是没有取消登记，然后再次登记。
This situation is generally indicative of a bug — that a listener was registered but not unregistered, and then was registered again.
您如果需要取消登记课程或更改日期，请提前一周通知我们，否则已付款项将不予退回。
We'll send the Confirmation Letter to you by fax ore-mail in a week after receiving the payment.
对于抛出未检测异常的侦听器，您可能想自动对它取消登记，毕竟，抛出未检测异常就证明侦听器坏掉了。
You might even want to automatically unregister a listener that throws an unchecked exception; after all, this is evidence that the listener is broken.
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