def to_xml(parser)
      n = parser.new_node('string')
      n = parser.append_node(n, parser.new_text(@value)) unless @value.nil?
      n
    end
def to_xml(parser)
      n = parser.new_node('string')
      n = parser.append_node(n, parser.new_text(@value)) unless @value.nil?
      n
    end
convert to XML