倒数到零或规定的时间 count down
此规则的例外情况是当值向下舍入到零时。
An exception to this rule is when a value is rounded down to zero.
当未完成的任务的总数减少到零时，join() 就会结束阻塞状态。
When the count of unfinished tasks drops to zero, join() unblocks.
贴现意味减少所有的计算值到当前值或零时的现值。
Discounting means to reduce all calculations to the present time or present worth at time zero.
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