How to select a document element from the input xml file using xslt?

Hello,
I am using a stylesheet for modifications of the input xml file to the
different format of the xml file.
My output file should have a document element, the same as in the input
file. How to copy a document element? I can have input XMLs with
different document elements. I tried to do something like this, but it
doesn't work:

<xsl:choose>
        <xsl:when test="count(//Delete)  &gt; 0">
                <xsl:output method="xml"
doctype-system="http://delete.dtd"/>
        </xsl:when >
        <xsl:when test="count(//Delete) = 0">
                <xsl:output method="xml"
doctype-system="http://update.dtd"/>
        </xsl:when>
</xsl:choose>
Please, help

Thank you, 

Alla Belkin