Question: is it at all possible to 'pre-proccess' in XSLT?

with preprocessing i mean updating the (in memory representation) of the source tree.

is this possible, or do i need to do multiple transforms for it.

use case:
we have Docbook reference manuals for out clients but for certain clients these need different 'skins' (different images etc). so what i was hoping to do is transform the image *fileref* path depending on a parameter. then apply the rest of the normal Docbook XSL templates.

Answer: 
Expanding on Eamon's answer...

In the case of either XSLT 1.0 or 2.0, you'd start by putting the intermediate (pre-processed) result in an <xsl:variable> element, declared either globally (top-level) or locally (inside a template).

```
<xsl:variable name="intermediate-result">
  <!-- code to create pre-processed result, e.g.: -->
  <xsl:apply-templates mode="pre-process"/>
</xsl:variable>

```

In XSLT 2.0, the value of the $intermediate-result variable is a node sequence consisting of one document node (was called "root node" in XSLT/XPath 1.0). You can access and use it just as you would any other variable, e.g., select="$intermediate-result/doc"

But in XSLT 1.0, the value of the $intermediate-result variable is not a first-class node-set. Instead, it's something called a "result tree fragment". It behaves like a node-set containing one root node, but you're restricted in how you can use it. You can copy it and get its string-value, but you can't drill down using XPath, as in select="$intermediate-result/doc". To do that, you must first convert it to a first-class node-set using your processor's node-set() extension function. In Saxon 6.5, libxslt, and 4xslt, you can use exsl:node-set() (as in Eamon's answer). In MSXML, you'd need to use msxsl:node-set(), where xmlns:msxsl="urn:schemas-microsoft-com:xslt", and in Xalan, I believe it's called xalan:nodeset() (without the hyphen, but you'll have to Google for the namespace URI). For example: select="exsl:node-set($intermediate-result)/doc"

XSLT 2.0 simply abolished the result tree fragment, making node-set() unnecessary.
Maybe you should use XSLT "OOP" methods here. Put all the common templates to all clients in a stylesheet, and create an stylesheet for each client with specific templates overriding common ones. Import the common stylesheet within the specific ones with xsl:import, and you'll do only one processing by calling the stylesheet corresponding to a client.