conditional inclusions

Hi there,
can one of you guys explain to us something we call "conditional inclusions".
say what?
let me try to explain our problem.
Let's say I have an XMl file which has an element such as:
<include url="file.xml"/>
This file would be part of the "input tree".
You would probably say that xsl:include would do the trick, yes but that's
not my question.
We want to make these inclusions conditional, and than work on that result
tree.
<if include="1">
<then>
<include url="file1.xml"/>
</then>
<else>
<include url="file2.xml"/>
</else>
</if>
We're gonna use it client -side, so preferably we want to use only one
stylesheet.
So in other words:
The xml we want to work with has a "tree" that is dynamically altered by
the choice of inclusion. This tree should be the basis for further
transformation 2 HTML ( preferably in the same xslt script!).
Maybe i'm missing something, but I was under the impression that the "tree"
that's the result of the inclusion, would automatically be outputted in the
output file. I do not want this, I want make html of that tree, not just
output the tree.
Erm hope this is clear to somebody..
RH
---------||-------||-----------
Ronald Heller
Salience BV
Villawal 21
3432 NX Nieuwegein
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fax: 030 6056 324
mob.: 06 20 41 25 77
ronald@xxxxxxxxxxx
www.salience.nl