>  <xsl:call-template name="$v1"/> <!-- this causes a trouble to XT -->

You can't do that. The name has to be a `qname'. Variable values
are never qnames. The value of your $v1 in this case is a result tree
fragment consisting of a text node with value `called' that is not the
same as the name called. For similar reasons as
select="called" isn't the same as select="'called'".

You have to do
<xsl:choose>
 <xsl:when test="$v1='called'">
   <xsl:call-template name="called"/>


So your input should cause trouble for any xsl processor.

David


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