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Introduction to XML For Web Developers
Optional Children  

Using the "?" character specifies that the element named is optional. Thus, in the following code snippet, we specify that every CONTACT must have a NAME and either a PHONE or EMAIL and may have an optional ADDRESS child.

	<!ELEMENT CONTACT (NAME, (PHONE | EMAIL), ADDRESS?)>
	<!ELEMENT NAME (#PCDATA)>
	<!ELEMENT PHONE (#PCDATA)>
	<!ELEMENT EMAIL (#PCDATA)>
	<!ELEMENT ADDRESS (STREET+, CITY, STATE, ZIP, COUNTRY?)
	<!ELEMENT STREET (#PCDATA)>
	<!ELEMENT CITY (#PCDATA)>
	<!ELEMENT STATE (#PCDATA)>
	<!ELEMENT ZIP (#PCDATA)>
	<!ELEMENT COUNTRY (#PCDATA)>

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