docs: function groupHighlights(highlights) {
      let order = [],
          chunks = {},
          grouped = [];
  
      highlights.forEach(function (hl) {
          let timestamp = hl.getAttribute(TIMESTAMP_ATTR);
  
          if (typeof chunks[timestamp] === 'undefined') {
              chunks[timestamp] = [];
              order.push(timestamp);
          }
  
          chunks[timestamp].push(hl);
      });
  
      order.forEach(function (timestamp) {
          let group = chunks[timestamp];
  
          grouped.push({
              chunks: group,
              timestamp: timestamp,
              toString: function () {
                  return group.map(function (h) {
                      return h.textContent;
                  }).join('');
              }
          });
      });
  
      return grouped;
  }