internal subset of DTD lost in output - but want to keep it

I have a document that has a doctype with public ID , system ID and an
internal subset of DTD.

When I do XLST translation with xml output and specify System ID and public
ID:
<xsl:output method="xml"
     doctype-public="...."
     doctype-system="...."
     indent="no" 
     encoding="ISO-8859-1" />,

the internal subset of DTD is lost in the output document.

Is this expected behavior? Is there a way to keep the internal subset of
DTD?

Thanks.

Peiyun Jiang

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