eliminate doctype statement

Is it possible to eliminate the doctype declaration at the beginning
of the xhtml output using docbook xsl 1.69? If so, how? Using
<xsl:output doctype-public"" doctype-system=""/> does not work; the
doctype just has "" for its' public and system identifiers.

The reason I ask is that my XHTML isn't validating due to the
inclusion of MathML. The only DTD that includes both is one from the
W3C that has both element sets in the same namespace, which docbook
xsl doesn't produce.

Thank you for your time,
--
http://chris.chiasson.name/