> and what about that ?
> 
> <xsl:variable name="concat">
> <xsl:sequence>
>   <xsl:value-of select="$a"/>
>   <xsl:value-of select="$b"/>
>   <xsl:value-of select="$c"/>
> </xsl:sequence>

xsl:sequence is no longer allowed to take child instructions in this way,
see:

http://www.w3.org/TR/xslt20/#element-sequence

It was allowed at one time, but the detailed specification led to too many
problems.

Michael Kay
http://www.saxonica.com/