code: func (h headerBatch) Less(i, j int) bool {
  	return bytes.Compare(h[i].hash[:], h[j].hash[:]) < 0
  }
docs: // Less reports where the entry with index i should sort before the entry with
  // index j. As we want to ensure the items are written in sequential order,
  // items with the "first" hash.
  //
  // NOTE: This is part of the sort.Interface implementation.