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><c> is a valid document</c></doc>
9