Writing an Var into a file

Hello List,

I have the Var calles $This.

For debuging reasons I want to output the variable into a output XML file. 
The XML file is not the same as the general sytelsheets output. In the end I
want to have two files. The general output, and the XML file including
"$This".

I was thinking about using something similiar to this:
 <xsl:copy-of select="document('This.xml')/$This)"/>

But that did not work.

Do You have an idea of how I could copy $This inot this.xml?

Thanks!!!