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</c><d> valid document</d></doc> 10