Re: select 1st occurence of element tag name

What
I need is a select construct which selects the first occurence of an
element tag name (tag name is not fixed) for all direct children of the
root.

Sounds like a grouping problem. There are several approaches,
I think Muenchean grouping is the simplest way here:

 <xsl:template match="/*">
   <xsl:apply-templates select="*[generate-id()=
     generate-id(key('name',name())[1])]/>
 </xsl:template>

See Jeni's page and the XSL FAQ for more hints:
 http://www.jenitennison.com/xslt/grouping/index.html