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