系统必须处理三类外部实体：生产线上的称重机、工人的控制台、审计日志。
The system must deal with three classes of external entities: the weighing machines on the production lines, the worker's console, and the audit log.
从短期来看，市场是一台投票机，但从长期来看，市场是一台称重机。
"In the short run, the market is a voting machine, but in the long run, it is a weighing machine." — Benjamin Graham, the Intelligent investor.
用一台轻便的弹簧秤称重。
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