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>
10