Subtitles in postgresql docs SGML?

Folks,

Is there an "idiot's guide" to our SGML format somewhere?  I'm new to updating
the actual Postgresql docs, and new to SGML in general.

I'm trying to figure out how to add a "sub-title" and "sub-section" within a
section.  Or something similar.

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-Josh BerkusAglio Database SolutionsSan Francisco