用于解析 DTD 位置的目录条目
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<public
publicId="-//OASIS//DTD DocBook XML V4.5//EN" 4
uri="docbook45/docbookx.dtd"/>
<system
systemId="http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" 5
uri="docbook45/docbookx.dtd"/>
<system
systemId="docbook4.5.dtd" 6
uri="docbook45/docbookx.dtd"/>