Absolute URIs on the Network
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http://docbook.org/rng/4.2/docbook.rng
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http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd
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urn:publicid:-:OASIS:DTD+DocBook+XML+V4.2:EN
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http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl
These offer the most unambiguous identification.