<?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>