simplized convert

    def convert(content)
      config = ReVIEW::Configure.values
      config['builder'] = 'html'
      config['secnolevel'] = 0 # 見出し採番しないとして
      ReVIEW::I18n.setup(config['language'])

      builder = ReVIEW::HTMLBuilder.new
      book = ReVIEW::Book::Base.new('.')
      book.config = config
      compiler = ReVIEW::Compiler.new(builder)
      chap = ReVIEW::Book::Chapter.new(book, '20191201', '-', nil, nil) 
      chap.content = content # 書き出したのをFile.readでもいいとは思うけど
      location = ReVIEW::Location.new(nil, nil)
      builder.bind(compiler, chap, location)
      compiler.compile(chap)
      s = builder.result
      s = s.gsub('<span class="secno">chapterchapter_postfix</span>', '')
      s = s.gsub(/<span class="secno">\d+\.\d+chapter_postfix<\/span>/, '')
      s
    end