DTD Customisation

Hi there

I'm part of a development team who have been assigned the task of
developing a documentation procedure for our company. We plan to use
docbook but it looks like we'll need to customise the DTD slightly. I
understand it's possible to create a customisation layer for the DTD to
allow this.

Our documents will have a strict structure as follows:

<book>
  |___<preface>
  |___<chapter>
  |     |
  |     |_<section role="topic">
  |     |  |_<section role="overview">
  |     |  |_<section role="detail">
  |     |  |_<section role="wrapup">
  |     |  
  |     |_<section role="topic">
  |     ...
  |
  |___<chapter>
  ...
  |___<appendix>
  |___<index>

ie: Any number of topics inside any number of chapters but each topic
MUST have the 3 children "overview", "detail" & "wrapup". This is
because different, related information must go in each of these with the
possibility of a sub-set being output to PDF when published.

What mods to the DTD would I need to make to ensure that a Validator
will only accept documents in this format?

Tia.
MikeD
-- 
  Mike Devlin