Help requested with XInclude and fileref resolution problem

[Taro Ikai <tikai@ABINITIO.COM>]

I have a modularized doc set whose directory structure 
looks like this:

  docroot/sub1/figures
  docroot/sub2/figures
  ...
  docroot/sub(n)/figures

At the 'docroot' level, I have a document whose toplevel 
element is a <set>, which XIncludes the main document
from each of the sub(n) directories.

The <book> document in each of the sub(n) directory in turn
XIncludes many <chapter> documents in the given directory.

To enable partial build of the large doc set, I've set up
Makefiles in both docroot/ and each of its subdirectories.

In other words, I can process either docroot/set.xml that
XIncludes all <book>s:

  <xi:include href="sub1/book.xml"/>
  ...
  <xi:include href="sub(n)/book.xml"/>

or, docroot/sub(n)/book.xml that XIncludes all chapter of 
the given <book>

  <xi:include href="chapter1.xml"/>
  ..
  <xi:include href="chapter(n).xml"/>

This works well except for one problem. In a given chapter 
file, say,

  docroot/sub1/chapter1.xml

I have a <graphic> element to a file in the corresponding
subdirectory: docroot/sub1/figures/foo.bmp:

  <graphic fileref="figures/foo.bmp"/>

This works well when I process docroot/sub1/book.xml.
But not when I process docroot/set.xml.

When the whole set is processed, the @fileref value
remains "figures/foo.bmp" instead of "sub1/figures/foo.bmp",
and the graphic does not show.

How should I solve this problem?

Taro