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Occurrence Indicators
In the previous example, also note the asterisk appended to the model group. This is an occurrence indicator-a special symbol that qualifies the element type or model group to which it is appended, indicating how many times it may occur. There are three occurrence indicators:
? Means optional and at most one occurrence (zero or one occurrence)
* Means optional and any number of occurrences (zero or more occurrences)
+ Means at least one occurrence required (one or more occurrences)
Content models can also define an element type as containing element content, illustrated by the SGML declaration for a definition list (<DL>) under HTML 4:
<!ELEMENT DL - - (DT | DD)+>
The XML declaration for dl under XHTML is again only slightly different as it omits the minimization information and cases the elements differently.
<!ELEMENT dl (dt | dd)+>
