The parentheses aren't needed if the element entity is already in a list.
如果元素实体已经在列表中，就不需要括号。
Each base content element has an element entity to identify itself and its specializations.
每个基本内容元素都有一个元素实体来标识其自身及其专门化。
The content model should allow for the possibility that the element entity might expand to a list.
内容模型应该允许元素实体可以扩展成列表的可能性。
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