The toc-depth defn uses constants that would be useful
as parameters, such as %book-toc-depth% and
In addtition, it would be nice to if the parameter name
computed from the top-element name, so that a
stylesheet could just define %article-toc-depth%
to change toc-depth itself.
The semantic of such %*-toc-depth% would be "use it if
else use %default-toc-depth%.