First of all many thx to everybody ...
I apologize if I posted a non-correct mail to the list .
With your hints , it "almost" works :)

<a href="{/webenmr/calculation/webinfo/@workingdir}/{@filename}"> ...
</a>

<a
href="{ancestor::calculation/webinfo/@workingdir}/{@filename}">...</a>

Output a correct href , even in if there's a :
<xsl:value-of select="@filename}"/>
between the two <a> and </a> tags I get an empty page :)

I think you're having problems because you haven't understood the concept of
the context node/item, or the details of when and how it changes.