def to_classic_html
      buf  = ""
      buf << "<div class='slide "
      buf << classes    if classes
      buf << "'>\n"
      buf << content
      buf << "</div>\n"
      buf
    end
def to_classic_html
      buf  = ""
      buf << "<div class='slide "
      buf << classes    if classes
      buf << "'>\n"
      buf << content
      buf << "</div>\n"
      buf
    end
convenience helpers