The structure of an XML document consists of a strict hierarchy of elements with a single root. A document type definition (DTD) specifies all of the valid element names that are part of a particular XML vocabulary.
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(content1, content2) the element must contain the sequence content1
and content2.
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(content1 | content2) the element must either include content1 or
content2.
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content1? the element could contain content1. If content1
occurs, then only once.
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content1* the element can contain contentt1. If it occurs, it
can occur once or more frequently.
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content1+ the element must contain content1 at least once,
but it also can more than once occurs.
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