Note xslt has no access to the tags in the original document, and can't
directly generate any in the result.

> a structure like this:


it depends a bit how like is like, but

<xsl:template match="*">
<xsl:copy>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>

<xsl:template match="a">
  <xsl:apply-templates/>
</xsl:template>

<xsl:template match="a/text()">
<a>
  <xsl:apply-templates/>
</a>
</xsl:template>

would produce the output you show.  In general though, if you need to
group more than one node inside teh a elements, this is a "grouping"
problem and you should use one of teh grouping idioms for xslt1 or
xsl:for-each-group in xslt2.

David