Root element is purpose dependent
Schema based options in Docbook 5.x (RelaxNG) requiring an <info> child in 5.1.
<info>
No limitation in Docbook 4.x (DTD).
chapter
section (recursive), sect1, sect2, sect3, sect4, sect5
refsection (recursive), refsect1, refsect2, refsect3
set, book, part
acknowledgements, appendix, bibliography, colophon, dedication, glossary, index, para, preface, refentry, reference, setindex, toc
article
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