<?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>
<d> is a valid document</d>
</doc>