def create_from_xml(xml, pares)
      raise PaResMissing.new "(2500) PaRes argument can not be omitted." if pares.nil?
      @request_xml = REXML::Document.new xml
      REXML::XPath.first(@request_xml, "//ThreeDSecure").add_element("PaRes").text=pares
    end
def create_from_xml(xml, pares)
      raise PaResMissing.new "(2500) PaRes argument can not be omitted." if pares.nil?
      @request_xml = REXML::Document.new xml
      REXML::XPath.first(@request_xml, "//ThreeDSecure").add_element("PaRes").text=pares
    end
TODO function to create classes (Customer, CreditCard and Operation) from xml document