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Each DTD element must either be a container element, or be empty (a place holder). Container elements may contain text, child elements, or a mixture of both.
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DTD also specifies the names of attributes, and dictates which elements they may appear in. For each attribute it specifies whether it is optional or required.
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DTD tree describing a book as containing a number of Chapter elements, with each chapter containing either a number of Paragraph elements, or a single Sections element
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A particular document tree has a node for each actual chapter and paragraph present, and may omit some of the optional elements
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