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.
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.
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.
通常Swing允许用跳格（Tab）、反向跳格和箭头键进行键盘导航。
Normally Swing allows keyboard navigation with the Tab, back-Tab, and arrow keys.
通常Swing允许用跳格（Tab）、反向跳格和箭头键进行键盘导航。
Normally Swing allows keyboard navigation with the Tab, back-Tab, and arrow keys.
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