这也避免了很多旅行的前进和后退。

It also avoided a lot of travel back and forward.

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下一页定义讯框间间距和后退演算法。

The next page defines interframe spacing and backoff.

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数据库模式中的每个变化都有自己的迁移对象，迁移对象包装了前进和后退。

Each change in the database schema has its own migration object, encapsulating a move up and a move down. Listing 1 shows an empty migration.

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