删除函数略微有些复杂，因为我们还想删除正在删除的键的反向映射。

Deletions are a little more complex, because we also want to delete the reverse mapping for the key we're deleting.

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除此以外，它还定义了一个secondary_keys方法来获取反向映射键。

In addition, it defines a secondary_keys method to get just the reverse mapping keys.

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一个全局反向索引仅仅是一个设计，其中创建的映射的键是搜索关键词，且其值是包含属性与搜索关键词的键列表。

A global reverse index is simply a design where you create a map whose key is the search term, and the value is the list of keys containing an attribute with that search term.

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