>   About the way I use to have the same base URI in the temporary tree
> than in the source tree (of the identity copy), does this seem to be
> the right way?
> 
>     <xsl:variable name="tmp.tree">
>       <tree>
>         <xsl:attribute name="xml:base" select="base-uri()"/>
>         <xsl:apply-templates mode="the.mode"/>
>       </tree>
>     </xsl:variable>
> 

Yes, I think this works. You may be straying into code that's not often
visited, however.

Michael Kay
http://www.saxonica.com/