DOCBOOK-APPS: [PSGML] Splitting docbook files

Hi all,
I'm just getting up to speed with DocBook (via NWalsh's The Def Guide).

I can publish my document in rtf and html ok, but the sgm file is huge - and
I want to
break it up.

I've found how to insert it into my master as an entity, but
since the "child" sub-file can't contain a DTG I can't see how to get
emacs/psgml to allow me to process the DTD or allow my to insert elements.

I've set the sgml-parent-file parameter, hoping that this would make emacs
pull the DTD from the parent file, but this didn't happen.

I guess I could try inserting the DTD reference into the child-file and
pressing "Parse DTD" from the menu bar - then commenting it out again before
I do a "real" publish, but I'm hoping there's a better way of doing it.

Can anybody give me some direction in this please?

Thanks,

Ian