xml - 创建 XML 文档类型定义时，如何指示在一组标签中，一个标签必须具有特定属性？

Question

我正在创建一个 XML DTD，我有一组标签，只有一个，必须包含一个属性。我如何在 DTD 中强制执行此操作？想想一个多项选择题/答案组。见下文:

<group>
    <question>This is a question</question>
    <question>This is another question</question>
    <question answer="true">This is another question, but this is the correct choice</question>
</group>

Answer 1

一种可能性是声明一种新的元素并且只允许它一次:

<!ELEMENT group (question*,questionWithAnswer,question*)>
<!ELEMENT question (#PCDATA)>
<!ELEMENT questionWithAnswer (#PCDATA)>

<!ATTLIST questionWithAnswer type (true|false) #REQUIRED>