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