Little black tags in http://www.docbook.org/tdg/en/html/ ...

Hi!

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http://www.docbook.org/tdg/en/html/ has littlr black tags for some of
the "DocBook Element Reference" entries, for example
"bibliocoverage<[4.2]> - The spatial or temporal coverage of a document"
where "<[4.2]>" is a small image (with the alternative text "[4.2]").
How was this feature implemented in DocBook ?

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Bye,
Roland

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