code: func (f Index) Count() (count int, err error) {
  	it := f.db.NewIterator()
  	defer it.Release()
  
  	for ok := it.Seek(f.prefix); ok; ok = it.Next() {
  		key := it.Key()
  		if key[0] != f.prefix[0] {
  			break
  		}
  		count++
  	}
  	return count, it.Error()
  }
docs: // Count returns the number of items in index.