1  <?xml version="1.0"?>
2  <!DOCTYPE doc [
3  <!ELEMENT doc ((a | b) , (c | d))+>
4  <!ELEMENT a (#PCDATA)>
5  <!ELEMENT b (#PCDATA)>
6  <!ELEMENT c (#PCDATA)>
7  <!ELEMENT d (#PCDATA)>
8  ]>
9  <doc><b>This</b><c> is a valid document</c></doc>