<?xml version="1.0"?>
<!DOCTYPE doc [
<!ELEMENT doc ((a | b) , (c | d))+>
<!ELEMENT a (#PCDATA)>
<!ELEMENT b (#PCDATA)>
<!ELEMENT c (#PCDATA)>
<!ELEMENT d (#PCDATA)>
]>
<doc><b>This</b><c> is a valid document</c></doc>
