I believe that we have both industry experience AND a formal model.
The industry experience is that people can build compound schemas by
combining schema parts through *parameter* entities. I highlight the word
*parameter* because it is the hint to how a more robust solution must
work: it must parameterize content models and schema fragments. The
problem with parameter entities is that they are way too flexible because
they work at a textual level. We have the right model, but the wrong
mechanism. So to move beyond that, we must implement parameterization in a
more structured sense.
-- The so-called "module proposal" did the same thing with SGML DTD syntax. (it uses a not-sufficient-constrained variety of parameter entities, however).http://www.ornl.gov/sgml/wg8/document/1987.htm http://www.ornl.gov/sgml/wg8/document/1982.htm
--The formal model is the forest automata theory. See:
http://lists.w3.org/Archives/Public/www-html/1998Mar/0017.html
This post shows how to do parameterization and composition.
Paul Prescod - http://itrc.uwaterloo.ca/~papresco
The past is inaccurate. Whoever lives long enough knows how much what he had seen with his own eyes becomes overgrown with rumor, legend a magnifying or belittling hearsay. "It was not like that at all!" -- he would like to exclaim, but will not, for they would have seen only his moving lips without hearing his voice. - Czeslaw Milosz (translated)