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http://www.w3schools.com/dtd/dtd_intro.asp
Document Type Definition (DTD)
A Document Type Definition (DTD) defines the legal building blocks of an XML
document. It defines the document structure with a list of legal elements and
attributes.
A DTD can be declared inline inside an XML document, or as an external
reference.
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Internal DTD Declaration
If the DTD is declared inside the XML file, it should be wrapped in a DOCTYPE
definition with the following syntax:
<!DOCTYPE root-element [element-declarations]>
Example XML document with an internal DTD:
<?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend</body>
</note>
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