Option to map comments into annotations (or maybe should be separate program).

Output overridden definitions.

Try to preserve marked sections.

Annotation for included sections (d:condition="flagName")

When inlining attlist decls, comments should be inlined also.  Maybe
inlining attlist decls should be separate later transformation.  Also
need to fix XSD output to deal with such comments.

Choose start elements based on presence of namespace declarations.

An attribute group that contains only namespace declarations should
disappear completely.
