Copy the Document Type Definition of XML file

I'm trying to copy resp. to modify an XML document via XSL. Here I use
the usual identity template:

<xsl:template match="@*|node()">
  <xsl:copy>
    <xsl:apply-templates select="@*|node()"/>
  </xsl:copy>
</xsl:template>

Unfortunately the DTD definition of the source document is not copied at
this way.