可以指定基本单词、类单词和修饰词是必需的还是可选的，以及这些元素出现的次序。
You can specify whether prime words, class words, and modifiers are mandatory or optional, and the order in which these elements should occur.
这些规则定义哪些元素是有效的、文档中元素的次序、属性的数据类型、一个元素是必需的还是可选的等等。
These rules describe, for example, which elements are valid, the order of elements within a document, the data types of attributes, whether an element is required or optional, and so on.
Control - flag：这指定任何特定模块是必不可少的、必需的、可选的还是足够的，从而定义模块的行为。
Control-flag: This defines the behavior of stacking modules by specifying whether any particular module is requisite, required, optional, or sufficient.
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