function makeSimpleLiveManifestText(lines, updateTime, duration) {
    const updateAttr = updateTime != null ?
        'minimumUpdatePeriod="PT' + updateTime + 'S"' : '';
    const durationAttr = duration != undefined ?
        'duration="PT' + duration + 'S"' : '';
    const template = [
      '<MPD type="dynamic" %(updateAttr)s',
      '    availabilityStartTime="1970-01-01T00:00:00Z">',
      '  <Period id="1" %(durationAttr)s>',
      '    <AdaptationSet mimeType="video/mp4">',
      '      <Representation id="3" bandwidth="500">',
      '        <BaseURL>http://example.com</BaseURL>',
      '%(contents)s',
      '      </Representation>',
      '    </AdaptationSet>',
      '  </Period>',
      '</MPD>',
    ].join('\n');
    const text = sprintf(template, {
      updateAttr: updateAttr,
      durationAttr: durationAttr,
      contents: lines.join('\n'),
      updateTime: updateTime,
    });
    return text;
  }
Makes a simple live manifest with the given representation contents.

@param {!Array<string>} lines
@param {number?} updateTime
@param {number=} duration
@return {string}