str = Nokogiri::XML::Builder.new { |xml|
          xml.REQUEST(:key => @key,
                      :fmt => @pos[:format],
                      :encoding => @pos[:encoding]) {
            xml.TRANSACTION {
              xml.FIELDS {
                req.fields.each { |field_name|
                  xml.send(field_name.to_s.upcase, req.send(field_name))
                }
              }
            }
          }
        }.to_xml#doc.to_xml



# works!