How to create processing instruction when writing XML using libxml-ruby

Actually I didn't find the way. So I had to use libxslt-ruby and
apply a stylesheet simply inserts PI.

require 'libxslt'
if @xslt
filter = <<-END_OF_XSLT
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<xsl:processing-instruction name="xml-stylesheet"
>type="text/xsl" href="#{@xslt}"</xsl:processing-instruction>
<xsl:copy-of select="."/>
</xsl:template>
</xsl:stylesheet>
END_OF_XSLT
stylesheet = LibXSLT::XSLT::Stylesheet.new(XML::Document.string(filter))
@xdoc = stylesheet.apply(@xdoc)
end