def to_xml_string(str = "")
      author = @comments.authors[author_index]
      str << ('<comment ref="' << ref << '" authorId="' << author_index.to_s << '">')
      str << '<text>'
      unless author.to_s == ""
        str << '<r><rPr><b/><color indexed="81"/></rPr>'
        str << ("<t>" << ::CGI.escapeHTML(author.to_s) << ":\n</t></r>")
      end
      str << '<r>'
      str << '<rPr><color indexed="81"/></rPr>'
      str << ('<t>' << ::CGI.escapeHTML(text) << '</t></r></text>')
      str << '</comment>'
    end
serialize the object
@param [String] str
@return [String]