external entity declaration in external file?

[Paul Reavis <preavis@partnersoft.com>]

I've got a large (ca. 1000 page) SGML document that I'm reorganizing.
One thing that has always bugged me is that I can't figure out a way to
split up external file inclusion. What I'd like to do is have several
subdirectories, and have the top-level document include files in these
to fill out the <!ENTITY> declaration section in the DTD statement.

For example, what I'm having to do currently is something like:

<!DOCTYPE set PUBLIC "-//Davenport//DTD DocBook V3.1//EN" [

<!ENTITY Book1Chapter1 SYSTEM "Book1/Chapter1.sgml">
<!ENTITY Book1Chapter2 SYSTEM "Book2/Chapter2.sgml">
<!ENTITY Book1Chapter3 SYSTEM "Book3/Chapter3.sgml">
<!ENTITY Book2Chapter1 SYSTEM "Book1/Chapter1.sgml">
<!ENTITY Book2Chapter2 SYSTEM "Book2/Chapter2.sgml">
]

But what I'd like to do is something like:

<!DOCTYPE set PUBLIC "-//Davenport//DTD DocBook V3.1//EN" [

<!ENTITY Book1Files SYSTEM "Book1/files.sgml">
<!ENTITY Book2Files SYSTEM "Book2/files.sgml">
&Book1Files;
&Book2Files;
]

This would simplify things and also allow me to define alternate
top-level documents, say if I just wanted to produce the printed version
of one of the books:

<!DOCTYPE book PUBLIC "-//Davenport//DTD DocBook V3.1//EN" [

<!ENTITY Book1Files SYSTEM "Book1/files.sgml">
&Book1Files;
]

Is there any facility like this? Are there better ways to organize the
<!ENTITY> declarations so they aren't such a pain?

-- 

Paul Reavis                                      preavis@partnersoft.com
Design Lead
Partner Software, Inc.                        http://www.partnersoft.com