当一个人收到电子邮件时，他们必须分离电子表格，编辑它并将其保存。
When a person receives the email, they must detach the spreadsheet, edit it, and save it.
持久性：用户必须保存他们的工作，并在以后再次装载它，所以需要一个持久性机制。
Persistence: Users must be able to save their work and load it again later, so a persistence mechanism is needed.
开发人员必须表明他们希望实现“深度”拷贝，以便完整地保存对象及相关内容。
Developers must indicate they intentionally desire a "deep" copy in order to save the object, its relatives, and so on fully.
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