OLINKDB.XML why are enitties needed?

Hi,

I just made our CHMs cross-linkable (olinks) - _really_ a nice job, I
had lots of fun =)

Well it works, but there is something I don't understand:

The olinkdb.xml file references the single target database files with an
entity.

(...)
<!ENTITY sf SYSTEM "somefile.tdb">
(...)
<document targetdoc="BOOK_SOMEFILE">&sf;</document>

i wrote a script to output olinkdb.xml and put the filenames directly in
the document element instead of declaring an entity.

<document targetdoc="BOOK_SOMEFILE">somefile.tdb</document>

But it doesn't work this way! Why? I thought entities are just replaced
by the parser in a first run, so where is the difference?
is there a way to create entity declaration with xslt? So that I could
rewrite my script?


Thx in advance,

Sebastian Fey