# File lib/iq_rdf/document.rb, line 161
    def to_xml
      xml = Builder::XmlMarkup.new(:indent => 2)
      xml.instruct!
      opts = {}
      @namespaces.values.each do |namespace|
        opts[namespace.token == :default ? "xmlns" : "xmlns:#{namespace.token.to_s}"] = namespace.uri_prefix
      end
      opts["xml:lang"] = @document_language if @document_language

      xml.rdf(:RDF, opts) do
        @nodes.each do |node|
          node.build_xml(xml)
        end
      end
      xml.target!
    end