Registers with Subtitles

If the register is divided into different subsections, these are structured by means of several <div> elements. 

<back>
  <div n="1" type="index">
  <head>[register title]</head>
    <div n="2">
    <head>[subtitle 1]</head>
      <list>
      <item>[keyword]
          <ref>[page number]</ref>
        </item>
      </list>
    </div>
    <div n="2">
    <head>[subtitle 2]</head>
      <list>
        <item>[keyword]
        <ref>[page number]</ref>
        </item>
      </list>
    </div>
  </div>
</back>