Any way to keep <!DOCTYPE...> definition int output of XSLT transformation?

I'm using XSL to transform a not-so-valid Docbook document into a valid 
Docbook document.

The not-so-valid document starts with a correct <!DOCTYPE ...> definition 
for Docbook.

When I run this document through XSLT (xsltproc program to be exact), the 
DOCTYPE definition 
gets erased.

Is there a way to keep the DOCTYPE definition in the output XML file? 

The only other alternative I can think of is to use <xsl:text> and print 
the DOCTYPE 
definition at the beginning of the document.

Taro