The PROBLEM occurs, when I try to process this node-set by

> a template that matches "tool". The template is ignored and
> no tool is matched!!

First, out put a xsl:copy-of of the variable to see whether
it contains the value you expected.
Second, RTFs are tricky. Check whether you have a template
matching "/". In most cases, you are better off using
 select="xx:node-set($foo)/*"
rather than
 select="xx:node-set($foo)"
If this still doesn't fix your problem, check whether
you have set a default namespace for the style sheet.