code: func (i *Index) ReferenceStats(id int) (stats index.ReferenceStats, ok bool) {
  	s := i.idx.Refs[id].Stats
  	if s == nil {
  		return index.ReferenceStats{}, false
  	}
  	return index.ReferenceStats(*s), true
  }
docs: // ReferenceStats returns the index statistics for the given reference and true
  // if the statistics are valid.